High-Temperature Components

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a high-temperature component made of a refractory metal or refractory metal alloy having the features of the preamble of claim 1 and to a method for manufacturing a high-temperature component. [Background technology]

[0002] In many components used in high-temperature applications, such as heating conductors for physical or chemical vapor deposition, electrodes in gas discharge lamps, or rotating X-ray anodes, the heat generated in the component is released into the environment by thermal radiation. The energy released is proportional to the thermal emissivity of the emitting surface. This value indicates how much radiation an object emits compared to an ideal black body. The higher the thermal emissivity of a surface, the greater the thermal radiation power the object can emit through this surface.

[0003] The same is true for the absorption of thermal radiant power, since the emissivity and absorptivity of an object are proportional, so an object with a high thermal emissivity will also absorb more radiant power than an object with a lower thermal emissivity.

[0004] For technical surfaces where heat transfer is desired by radiation, efforts are made to achieve the highest possible thermal emissivity. Thanks to improved radiation capabilities, the same radiation output can be emitted at a lower component temperature. Therefore, to achieve a given radiation output, a component with a high thermal emissivity can be operated at a lower operating temperature than a corresponding component with a lower thermal emissivity. Lower component temperatures generally have a favorable effect on the component's service life and process stability.

[0005] Various approaches to achieve higher thermal emissivity are known from the prior art, one of the known approaches aims at enlarging the microscopic surface of the emitting face.

[0006] One example is US Patent No. 5,629,663, which describes a heating conductor having a porous sintered coating of tungsten applied by a slurry method. The porous sintered coating of tungsten can improve the thermal emissivity to about 0.34 in the wavelength range of 1700-2500 nm, compared to the thermal emissivity of a smooth tungsten surface at room temperature in this wavelength range of about 0.16.

[0007] Patent Document 2 describes an anode for a high-pressure gas discharge lamp with a metal coating having a dendritic structure. The needle-like crystallites of the dendritic structure also increase the surface area of ​​the anode. A thermal emissivity of up to 0.8 should be achievable. However, the dendritic structure is very complex and expensive to manufacture.

[0008] A common drawback of the aforementioned coating solutions with structures in the low μm range is the degradation of the coating over its service life. In particular, at operating temperatures above 1500°C, the sintering process causes a constant decrease in surface area and, therefore, thermal emissivity. Structuring the surface on a scale of several hundred μm, for example by laser, in order to avoid the sintering process, is very cost-intensive.

[0009] Efforts are being made not only on geometric optimization of the surface but also on further development of suitable coating materials to increase the thermal emissivity.

[0010] Patent Document 3 describes a high-temperature component made of a heat-resistant metal having a coating containing tantalum nitride and / or zirconium nitride and tungsten, with the tungsten content being 0 to 98 wt.% (weight percent). A thermal emissivity of up to 0.8 should be achievable.

[0011] Patent Document 4 discloses an electrode for a discharge lamp having a coating in which tungsten particles are embedded in a ceramic matrix layer.

[0012] Both of the aforementioned coatings have in common that they are not suitable for thermally highly loaded components such as heating conductors used in coating equipment, especially MOCVD (Metal Organic Chemical Vapor Deposition) equipment, which are exposed to operating temperatures above 2000°C.

[0013] Patent Document 5 describes electrodes for high-pressure gas discharge lamps coated with rhenium. Rhenium has a higher thermal emissivity than tungsten, but is very expensive. For cost reasons, tungsten can be added to the coated rhenium. The thermal emissivity of the resulting mixture is reduced by the incorporation of tungsten compared to pure rhenium. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 2014 / 023414(A1) Brochure [Patent Document 2] European Patent No. 1019948(B1) [Patent Document 3] International Publication No. 2018 / 204943(A2) Brochure [Patent Document 4] German Patent No. 102009021235 (B4) [Patent Document 5] US Patent Application Publication No. 2002 / 0079842(A1) Summary of the Invention [Problem to be solved by the invention]

[0015] It is an object of the present invention to further develop high temperature components and to provide methods for their manufacture, which preferably are distinguished by high thermal emissivity and are suitable for operating temperatures of around 2000°C or higher. [Means for solving the problem]

[0016] This problem is solved by a high-temperature component having the features of claim 1 and a method having the features of claim 10. Preferred embodiments are set out in the dependent claims.

[0017] The applications considered in connection with this application are typically those with operating temperatures of 1000 to 2500° C. or higher. This applies in particular to applications in lighting technology (e.g. electrodes of high-pressure discharge lamps), furnace technology (e.g. heating conductors, furnace fittings, charging devices, crucibles) and medical technology (e.g. rotating X-ray anodes).

[0018] Components that involve high operating temperatures are referred to in this application as high temperature components.

[0019] Refractory metals or refractory metal alloys are generally used for the above-mentioned high-temperature applications. In this application, refractory metals are understood to mean metals from groups 4 (titanium, zirconium, and hafnium), 5 (vanadium, niobium, and tantalum), and 6 (chromium, molybdenum, and tungsten) of the periodic table, as well as rhenium. Refractory metal alloys refer to alloys containing at least 50 at.% (atomic percent) of the element in question. These materials have, among other things, excellent dimensional stability at high service temperatures. The high-temperature components are based on refractory metals or refractory metal alloys. This means that the high-temperature components consist essentially of refractory metals, i.e., at least 50 at.%, preferably more than 95 at.%, of refractory metals. Particularly preferably, the high-temperature components consist entirely of refractory metals or refractory metal alloys and common impurities. The actual high-temperature components can be equipped with attachments. The high-temperature components can be, for example, part of composite components.

[0020] A particularly preferred material for high temperature components due to its heat resistance is tungsten or a tungsten alloy.

[0021] This class of high temperature components has a coating to increase the thermal emissivity, whether the coating is applied to the entire component or only to a portion of it.

[0022] According to the invention, the coating for increasing thermal emissivity consists essentially of tungsten and rhenium, where the proportion of rhenium is at least 55% by weight and the proportion of tungsten is at least 10% by weight (limits included), in other words, the proportion of rhenium is 55% to 90% by weight and the remainder is tungsten, preferably the proportion of rhenium is 60% to 85% by weight, particularly preferably the proportion of rhenium is 65 to 80% by weight.

[0023] Here, "essentially" means that the main components are tungsten and rhenium. The coating may contain small amounts of other components and common impurities. Impurities may include, for example, oxides, nitrides, or carbides, as well as metals such as molybdenum, iron, copper, tantalum, and niobium. However, the proportion of the main components, tungsten and rhenium, is preferably greater than 95% by weight, particularly greater than 98% by weight.

[0024] According to the invention, the tungsten and rhenium in the coating are at least partially present in the form of the cubic Re3W phase, i.e. the coating has at least 35% by weight, in particular at least 40% by weight, particularly preferably at least 50% by weight, and very particularly preferably at least 70% by weight of the cubic Re3W phase.

[0025] Re3W is an intermetallic phase with a cubic crystal system, and apart from the lattice constant, it has the same structure as cubic Re 0.75 W 0.25 In the context of the present invention, the Re3W phase corresponds to a cubic Re 0.75 W 0.25 It is also understood to be a phase.

[0026] The different phases in the tungsten-rhenium binary phase diagram can be seen in Figure 1. Phases in which the material exists in the form of a solid solution are shown in brackets, while phases without brackets are intermetallic phases. In contrast to solid solutions, they exhibit a lattice structure different from that of the constituent metals, in which there is mixed bonding between the individual metal atoms, consisting of metallic and subatomic or ionic bonding components. The Re3W phase (or Re 0.75 W 0.25 phase) is represented by χ in the phase diagram.

[0027] Surprisingly, it has been found that the thermal emissivity increases significantly when a minimal amount of the Re3W phase is present. The thermal emissivity of tungsten with a smooth surface is approximately 0.16 at room temperature in the wavelength range of 1700-2500 nm, and the corresponding thermal emissivity of rhenium is approximately 0.18. Thus, one skilled in the art (see also U.S. Patent No. 5,629,399) would expect the thermal emissivity to decrease as the rhenium percentage decreases. Surprisingly, however, this is not the case. The thermal emissivity for the claimed range of rhenium concentrations of 55-90 wt.% rhenium is significantly higher than would be expected for this rhenium concentration from linear interpolation between the thermal emissivities of tungsten and rhenium. When a very high percentage of the Re3W phase is present, thermal emissivity even higher than that of pure rhenium, i.e., values ​​greater than 0.18, can be achieved for certain rhenium concentrations. In applicant's testing, a maximum value for thermal emissivity was achieved between approximately 70-80 wt.% rhenium.

[0028] For quantitative phase analysis, a representative sample is removed from the coating, ground to a powder, and the resulting powder is analyzed by XRD (X-ray diffraction).

[0029] Applicants speculate that the special characteristics regarding thermal emissivity are due to the phonon spectrum of the intermetallic Re3W phase.

[0030] The thermal emissivity due to the material properties of the coating can further be increased by measures to enlarge the microscopic surface.

[0031] Preferably, the coating is formed porous. Here, porous means that the coating has a significant pore ratio, for example, more than 5%. Here, the pore ratio is understood to be the area ratio of pores in the total cross-sectional area, which is determined based on the representative cross-sectional area of ​​the coating sample. Due to the presence of pores in the volume of the coating, the surface of the coating is enlarged compared to the purely geometric surface, thereby further increasing the thermal emissivity. Porous coatings can be produced, for example, by powder metallurgy.

[0032] Alternatively, the surface of the high-temperature component to which the coating is applied may already be enlarged compared to a purely geometrical surface. In other words, in this variant, the surface of the high-temperature component is structured below the coating and thereby enlarged. The structuring can be carried out by mechanical, chemical or thermal methods. In this case, the coating itself is not necessarily porous. Pretreatment of the surface of the high-temperature component is particularly important for PVD (physical vapor deposition) coating methods.

[0033] According to a preferred embodiment, the coating is formed as a sintered layer. By sintered layer, it is understood that a layer is obtained by a powder metallurgical coating method. An example of a powder metallurgical coating method is slurry coating. After the coating material is actually applied in the form of particles, the layer application is solidified by sintering. Sintered layers are generally porous and have a rough surface.

[0034] The coating may be in the form of a PVD layer. In this case, the coating is produced on the surface of the high-temperature component using a suitable sputtering target in a physical vapor deposition process. PVD layers are generally smooth and dense, and therefore pore-free. To enlarge the surface, the surface of the high-temperature component may be structured by mechanical, chemical, or thermal methods before coating.

[0035] PVD and sintered layers can be easily distinguished as they have very different surface properties.

[0036] For manufacturing reasons, the sintered layer preferably has a thickness of 2 μm to 300 μm, more preferably 3 μm to 100 μm, and particularly preferably 5 μm to 50 μm.

[0037] In the case of PVD layers, the thickness may also be significantly smaller: typical thicknesses for PVD layers are between 10 nm and 4 μm.

[0038] The thickness of the coating is not critical to the function.

[0039] The coating is preferably formed on the coated side of the high-temperature component, meaning that the coating forms the outermost layer on the surface of the high-temperature component, which is intended to participate in the heat transfer by radiation when the high-temperature component is in use.

[0040] There may be further layers underneath.

[0041] According to a preferred exemplary embodiment, the high-temperature component is formed as a heating conductor. In this application, a heating conductor refers to a metal resistance heater such as those used in heat treatment equipment. The heating conductor may be formed from sheet metal, bar stock, stranded wire, bundled wire, or wire mesh. In the case of flat heating conductors, i.e., heating conductors whose basic shape is obtained from sheet metal, it may be desirable to provide a coating only on the side of the heating conductor that faces the interior of the furnace during operation of the heating conductor.

[0042] When used on a heating conductor, the coating has the advantage that a given heating output can be generated at a lower temperature due to improved heat radiation. Lower operating temperatures of the heating conductor are advantageous in terms of service life, for example, as they can reduce creep of the material.

[0043] The advantageous coating of heating conductors used in coating systems, especially MOCVD systems, is of particular interest. Due to the high operating temperatures of over 2000°C, there is a risk of evaporation of the heating conductor material and, therefore, contamination during the coating process. According to the current state of the art, these heating conductors are made of either tungsten or rhenium, with rhenium being the heating conductor subjected to the highest thermal loads. Both materials have low vapor pressures at high temperatures, but differ in thermomechanical properties. Therefore, for certain applications, more expensive rhenium is preferred over less expensive tungsten. The coating of the present invention can increase the degree of thermal radiation of tungsten heating conductors, thereby reducing their surface temperature, significantly expanding their application range. The rhenium- and tungsten-coated tungsten heating elements of the present invention are an economically very attractive alternative to heating elements made entirely of rhenium, which are correspondingly expensive. Thanks to their relatively high thermal radiation, they can be operated at relatively low temperatures for a given heating output.

[0044] Similarly, it is believed that the present invention will allow for the replacement of tungsten heating conductors in certain applications with molybdenum-based heating conductors coated with rhenium and tungsten according to the present invention.

[0045] Of course, it is also possible to provide a coating containing the Re3W phase on the rhenium heating conductor to enhance the degree of heat radiation, particularly in high temperature applications.

[0046] Although heating conductors are discussed here, the suggestions regarding substrates also apply to other high temperature components.

[0047] According to another embodiment, the high-temperature component is designed as an electrode of a high-pressure discharge lamp, in particular as an anode of a high-pressure discharge lamp. The coating of the electrode, in particular the anode, according to the invention allows the high-temperature component to radiate more heat during operation, which leads to a reduction in the component temperature and has a beneficial effect on the service life.

[0048] According to a further embodiment, the high-temperature component is configured as a crucible. Heat-resistant metal crucibles are used, for example, to melt aluminum oxide in the production of sapphire single crystals. For this purpose, the crucible is placed in a high-temperature furnace, where it is heated by a heating conductor via radiant heat. Heat transfer occurs primarily through the sides of the crucible, which absorb the radiant heat and transfer it to the object to be melted. The coating according to the present invention allows a large portion of the heat emitted by the heating conductor to be transferred to the crucible.

[0049] Preferably, the coating has a thermal emissivity of ε>0.6 measured over the wavelength range of 1700-2500 nm at room temperature, as explained in more detail below.

[0050] The invention also relates to a method for manufacturing a high temperature component. According to the invention, the method for manufacturing a high temperature component comprises the following steps: - providing a substrate for a high temperature component, and then i) - optionally increasing the surface area of ​​the substrate of the high temperature component; - coating the substrate with tungsten and rhenium by physical vapor deposition using a target material containing tungsten and rhenium, the target material having at least 35 wt% of the Re3W phase; or ii) - optionally increasing the surface area of ​​the substrate of the high temperature component; - coating the substrate with tungsten and rhenium by physical vapor deposition using a target material containing tungsten and rhenium; - heat treating the coated substrate at a heat treatment temperature of 500°C or higher, preferably 1000°C or higher, more preferably above 1800°C, in an inert or reducing atmosphere or under high vacuum to form the Re3W phase; - optionally, slowly cooling the coated and heat-treated substrate from the heat treatment temperature to 800°C; - quenching the substrate to room temperature at a cooling rate of more than 20 K / min to stabilize the Re3W phase; or iii) - coating the substrate by powder metallurgy with a powder mixture containing rhenium and tungsten having a molar ratio of 25 atomic % tungsten to 75 atomic % rhenium; - heat treating the coated substrate at a heat treatment temperature of 500°C or higher, preferably 1000°C or higher, more preferably above 1800°C, in an inert or reducing atmosphere or under high vacuum to form the Re3W phase; - optionally, slowly cooling the coated and heat-treated substrate from the heat treatment temperature to 800°C; - quenching the substrate to room temperature at a cooling rate of more than 20 K / min to stabilize the Re3W phase.

[0051] By substrate is understood the hot part before coating or the semi-finished product from which this part is produced.

[0052] Three different process variants are proposed: process variants i) and ii) are based on PVD methods, and process variant iii) is based on powder metallurgy methods.

[0053] According to method variants i) and ii), the surface of the substrate of the high-temperature component is first preferably pretreated so that the surface is enlarged compared to the geometrical surface. This "roughening" can be carried out by removing material at the surface, for example by structuring the surface by mechanical methods, such as (sand) blasting, chemical methods (e.g. etching or pickling), or thermal methods (e.g. laser structuring).

[0054] Alternatively or additionally, the surface enlargement can also be achieved by slurry coating. In the slurry method, powdered components are slurried in a liquid. The part (here, the substrate of the high-temperature part) can be coated with the resulting suspension, which generally also contains a binder, by dipping, spraying, brushing, or the like. After drying, the coating is generally sintered. The coating thus formed is generally porous and rough. This forms a favorable base layer for subsequent coatings. The slurry coating can be based on, for example, tungsten powder.

[0055] Subsequently, according to method variant i), tungsten and rhenium are applied to the substrate (with its surface enlarged, if desired) by physical vapor deposition. A target material containing tungsten and rhenium with a corresponding composition can be used as the source, with the desired Re3W phase already present in the target material in sufficient amounts. The desired rhenium proportion in the layer can be determined by appropriately selecting the target composition. In addition to this target material already containing a sufficient amount of Re3W phase and whose composition corresponds to the desired tungsten-rhenium content in the layer to be deposited, two or more target materials can alternatively be used, with one consisting predominantly or exclusively of the Re3W phase and one or more additional target materials consisting of tungsten and / or rhenium with the corresponding tungsten-rhenium composition being provided to adjust the desired tungsten-rhenium concentration.

[0056] Preferably, the target material comprises at least 35% by weight of the cubic Re3W phase, more preferably the proportion of the Re3W phase is at least 40% by weight, particularly preferably at least 50% by weight, and very particularly preferably at least 70% by weight.

[0057] As a result, the PVD coating has a rhenium proportion of 55% to 90% by weight, the remainder being tungsten, where the proportion of the Re3W phase is at least 35% by weight.

[0058] This method variant i) (PVD coating with Re3W phase) can be advantageous when it is desirable to avoid distortion of components with narrow component tolerances, i.e. the PVD coating is carried out at a relatively low temperature and does not require any heat treatment of the coating.

[0059] Method variant ii) is likewise a PVD coating method, differing from variant i) in that the Re3W phase is not necessarily present in the target material, but is formed subsequently only by heat treatment in the sputtered layer. For this purpose, the substrate coated by physical vapor deposition is subjected to annealing at a heat treatment temperature in the phase field of the Re3W phase. The heat treatment temperature is above 500°C, technically preferably above 1000°C, and even more preferably above 1800°C. The duration of the heat treatment depends on the heat treatment temperature.

[0060] At temperatures below 1000° C., the formation of the desired phase occurs very slowly due to insufficient reaction rates, which makes such heat treatment temperatures less attractive from a technological point of view.

[0061] Good results were obtained at a heat treatment temperature of about 1800° C. and a holding time of 20 hours. Those skilled in the art can obtain further suitable combinations of heat treatment temperature and holding time through experimentation.

[0062] The criterion in the selection of the heat treatment parameters is that the heat treatment achieves a Re3W phase content in the coating of at least 35 wt. %.

[0063] In particular, it is desirable that the heat treatment achieves a Re3W phase content in the coating of at least 40% by weight, particularly preferably at least 50% by weight, and very particularly preferably at least 70% by weight.

[0064] Here, the inert atmosphere is provided by an inert gas such as nitrogen or argon at a pressure of about 1 bar, and the reducing atmosphere is provided by hydrogen, for example. -3 ~10 -8 By the heat treatment, the tungsten and rhenium in the sputtered layer are at least partially converted into the intermetallic Re3W phase.

[0065] After heat treatment, the coated substrate is preferably slowly cooled from the heat treatment temperature to about 800° C., after which it is rapidly cooled to room temperature. Slow cooling to a temperature below the heat treatment temperature but still within the phase field of the ReW phase may be technically advantageous to protect the heat treatment equipment used.

[0066] However, it is equally possible to rapidly cool directly from the heat treatment temperature to room temperature.

[0067] Upon rapid cooling, the Re3W phase, which is metastable at room temperature, becomes kinetically stabilized.

[0068] In this application, slow cooling is understood to mean cooling on a time scale of several hours, corresponding to a cooling rate of 1 K / min to 10 K / min, typically less than 10 K / min.

[0069] In this application, rapid cooling is understood to mean quenching at a cooling rate typically in the range of 20 to 150 K / min, preferably more than 25 K / min, more preferably more than 50 K / min, particularly preferably more than 100 K / min.

[0070] Re3W is in a metastable phase below about 500°C and is kinetically stabilized by rapid cooling. Process variant ii) has the advantage over process variant i) that no Re3W-containing target material is required (although, of course, target materials already containing Re3W may also be used). The disadvantage is the need for an additional heat treatment step at a relatively high temperature.

[0071] According to process variant iii), the substrate is first coated by powder metallurgy with a powder mixture containing rhenium and tungsten (tungsten to rhenium molar ratio of about 1:3), followed by heat treatment (i.e., annealing to form the Re3W phase, quenching to stabilize the Re3W phase) as in process variant ii).

[0072] Here, rhenium-containing or tungsten-containing means that the powder contains rhenium or tungsten in metallic form. The powder mixture may contain other components, such as a binder, in addition to the two metals. The powder metallurgical process may be, in particular, a slurry process. The heat treatment solidifies the powder-metallurgically applied layer. Due to the relatively long process duration of about 20 hours (as an appropriate holding time for the example of a heat treatment temperature of about 1800°C), the tungsten-rhenium particles are partially converted into the intermetallic Re3W phase, although they may exist in solid solution form during the process duration of about 3 to 10 hours typically used for these metals. After the heat treatment, the coated substrate is optionally slowly cooled to 800°C and then quenched to room temperature. The heat treatment and cooling parameters correspond to those of method variant ii). The quenching kinetically stabilizes the Re3W phase, which is metastable at room temperature.

[0073] This method variant (powder metallurgical coating) has cost advantages over classical PVD methods. Furthermore, the layer thicknesses of the Re3W phase achieved are usually higher, which has a favorable effect on the long-term stability of the coating.

[0074] The present invention will now be described in more detail with reference to the following production examples and drawings. [Brief explanation of the drawings]

[0075] [Figure 1] 1 is a phase diagram for the binary tungsten-rhenium system. [Figure 2a] 1 is a scanning electron micrograph (cross section (fracture surface)) of a surface coated according to the present invention. [Figure 2b] 1 is a scanning electron micrograph (top view) of a surface coated according to the present invention. [Figure 2c] 1 is a scanning electron micrograph (cross section (fracture surface)) of a surface coated according to the present invention. [Figure 2d] 1 is a scanning electron micrograph (top view) of a surface coated according to the present invention. [Figure 3] 1 is a diagram with values ​​of thermal emissivity epsilon (ε) for different coatings. [Figure 4a] 1 is an X-ray diffraction diagram (XRD) of a layer according to the present invention and a layer produced by a conventional method. [Figure 4b] 1 is an X-ray diffraction diagram (XRD) of a layer according to the present invention and a layer produced by a conventional method. [Figure 5] 1 is a schematic diagram of a high-pressure discharge lamp as an exemplary embodiment of a hot component; [Figure 6] 1 is a heating conductor as an exemplary embodiment of a high-temperature component. [Figure 7] 1 is a crucible as an exemplary embodiment of a high-temperature part. DETAILED DESCRIPTION OF THE INVENTION

[0076] Manufacturing example I: To produce the high-temperature components according to Production Example I, tungsten substrates were coated with slurries of different powder mixtures. To do this, tungsten and / or rhenium powders were first weighed into a 2% by weight ethyl cellulose binder in ethanol to a total solids content of 50%. Stirring was carried out for 15 minutes at 1500 rpm using a Netzsch Multimaster.

[0077] Samples were prepared with the following layer compositions: 100% tungsten by weight 10% by weight rhenium, the rest tungsten 20% by weight rhenium, the rest tungsten 30% by weight rhenium, the rest tungsten 40% by weight rhenium, the rest tungsten 50% by weight rhenium, the rest tungsten 60% by weight rhenium, the rest tungsten 70% by weight rhenium, the rest tungsten 80% by weight rhenium, the rest tungsten 90% by weight rhenium, the rest tungsten 100% rhenium by weight.

[0078] The weight percentages given here relate to the weight of the solid components rhenium and tungsten, and also correspond to the weight percentages in the layer since the organic components are volatilized during heat treatment.

[0079] Subsequent spray coatings were 15 mg / cm 2 The coating was performed manually on a tungsten plate at a distance of about 20 cm until the target layer mass was reached. Drying was performed in room air.

[0080] The layers that had begun to dry were subsequently subjected to a heat treatment (annealing), which volatilized the organic components (e.g., binders) and solidified the layers. Each heat treatment was carried out at 1800 °C for 20 h under an argon (Ar) atmosphere. After the heat treatment, the coated substrates were gradually cooled to 800 °C over 10 h (corresponding to an average cooling rate of 1.67 K / min) and then quenched to room temperature within 20 min from about 800 °C (corresponding to an average cooling rate of about 40 K / min).

[0081] For comparison purposes, an additional sample of 80 wt% rhenium and balance tungsten was prepared similarly to the method described above, except that it was heat treated in an argon atmosphere at 1600°C for 6 hours instead of 20 hours.

[0082] Measurements of the thermal emissivity of the layers were performed at room temperature using a Solar 410 Reflectometer from Surface Optics Corporation in the wavelength range of 1700–2500 nm, since this infrared wavelength range is particularly relevant for evaluating the thermal emissivity of objects.

[0083] In the table below, the measurement results are also compared with known values ​​of the thermal emissivity of coatings known in the prior art, for example coatings with tantalum nitride according to US Pat. No. 5,629,999.

[0084] Selected results are summarized in Table 1 and a more detailed representation can be seen in the graph in Figure 1, where the thermal emissivity epsilon (ε) is shown as a function of rhenium content.

[0085] [Table 1]

[0086] Sample No. 1, a porous tungsten coating obtained using a 100% tungsten slurry, has a thermal emissivity of 0.34, and Sample No. 2, a porous rhenium coating obtained using a 100% rhenium slurry, has a thermal emissivity of 0.36. Sample No. 3 is a coating made from tantalum nitride, prepared according to the description in Applicant's U.S. Patent No. 5,499. It has a relatively high thermal emissivity of 0.89, but is only suitable for temperatures up to 1500°C. Sample No. 4, for comparison purposes, is a coating made of 80% rhenium and 20% tungsten, prepared by heat treatment at 1600°C for 6 hours, as described above. This sample contains primarily a tungsten / rhenium solid solution, with only a very small proportion of the Re3W phase, as will be explained in more detail below. It has a thermal emissivity of 0.35. Sample No. 5 is an 80% rhenium and 20% tungsten coating produced according to the instructions given above (heat treatment at 1800°C for 20 hours). The proportion of the Re3W phase is approximately 90% by weight. The thermal emissivity was determined to be 0.66.

[0087] Figures 2a-2d show scanning electron micrographs of sample No. 5. Figures 2a and 2b show images magnified 1000 times, while Figures 2c and 2d show images magnified 3000 times. Figures 2a and 2c show fracture surfaces perpendicular to the sample surface, while Figures 2b and 2d show plan views of the surface, i.e., the observation direction is perpendicular to the coating surface. In the fracture surfaces, the substrate 2, made of tungsten sheet metal, can be seen at the bottom of the image. Above it, the porous coating 3 can be seen. Porosity increases the microscopic surface area, further increasing the thermal emissivity.

[0088] Figure 3 shows a diagram of the measured thermal emissivity epsilon (ε) for the series of tests with different rhenium contents listed at the beginning. The rhenium content is plotted on the abscissa and the measured thermal emissivity epsilon (ε) is plotted on the ordinate. The dots in the diagram represent the respective measured values. Dashed line ε th (theoretical epsilon) indicates the thermal emissivity value that can be predicted by linearly interpolating the thermal emissivity from 100 wt% tungsten to 100 wt% rhenium. In particular, in the range of 50 wt% to 90 wt% rhenium, the measured thermal emissivity surprisingly does not follow this straight line ε th It can be seen that the thermal emissivity does not run along the Re3W phase, but is located above it, and in some places very significantly above it. The maximum thermal emissivity occurs in the range of 70-80 wt. % rhenium. Applicant's measurements show that the advantageous increase in the thermal emissivity can be attributed to the presence of the Re3W phase.

[0089] This is shown in Table 2, which shows the results of detailed quantitative phase analysis for samples with a rhenium content of 70 wt. % (Sample I) or 80 wt. % (Sample II). For quantitative phase determination, a portion of the coating of each sample was scraped off, ground into powder, and analyzed by XRD. For comparison, the measurements for samples (Sample Ia and Sample IIa) produced by conventional methods (i.e., heat treatment duration of 6 hours) are also listed.

[0090] [Table 2]

[0091] (W) and (Re) are both solid solution phases ((W) is a tungsten crystal with rhenium dissolved therein, and (Re) is a rhenium crystal with tungsten dissolved therein). W 0.5 Re 0.5 is an intermetallic phase, also referred to as the σ phase in the phase diagram. The amounts of the phases are expressed in weight percent.

[0092] The measurements show that the proportion of Re3W is significantly higher in the samples with the coating according to the invention, which are heat-treated for significantly longer periods, than in the samples produced with heat-treatment durations typically applied in the powder metallurgical processing of tungsten and rhenium. The proportion of Re3W is around 90 wt.% in both samples, i.e., Sample I (70 wt.% rhenium) and Sample II (80 wt.% rhenium), compared with 21.8 wt.% (Sample Ia) or 27.8 wt.% (Sample IIa) for the corresponding conventionally produced samples. The higher proportion of Re3W is also associated with a significantly higher thermal radiation coefficient.

[0093] Figures 4a and 4b show the X-ray diffractograms (XRD) of sample II (Figure 4a) and sample IIa (Figure 4b). In the diffractograms, the intensity values ​​are plotted as a function of the 2-theta deviation angle (2-theta range 30-65), and the measured reflections (peak values) are assigned to the phases present. In sample II with the coating according to the invention, the proportion of Re3W predominates.

[0094] Table 3 shows the heat resistance of the samples of the present invention. The measured values ​​of the thermal emissivity as a function of temperature are shown when the samples were subjected to a thermal stress test. The samples were annealed at this temperature for 1 hour.

[0095] [Table 3]

[0096] A series of tests showed that the thermal emissivity did not decrease significantly at T = 2000 °C (from 0.66, the value for the sample before the thermal stress test, to 0.65), and the coating did not deteriorate. At T = 2200 °C, a decrease in thermal emissivity of approximately 12% was observed. The material withstands high thermal loads, but the porous layer begins to sinter somewhat. Nevertheless, the high thermal emissivity is maintained even at this high temperature. Thus, the coating according to the present invention can withstand loads above 2000 °C and can therefore be used to heat filaments in MOCVD equipment.

[0097] Production example II: An alternative variation for producing the coating is based on physical vapor deposition. In this example, a tungsten plate was first coated with a conventional 100% tungsten slurry layer, which served to enlarge the surface. On top of this layer, a layer with a thickness of approximately 4 μm and containing the Re3W phase was sputtered using a target with a phase of approximately 98% Re3W. The resulting layer contained approximately 75% rhenium by weight. The measured thermal emissivity did not fully approximate the value of Example I, since some radiation exchange also occurred through parts of the porous tungsten structure not covered by the PVD coating.

[0098] An example of application to this high-temperature component will be described below with reference to FIGS.

[0099] FIG. 5 shows a schematic representation of a high-pressure discharge lamp 6. During operation, a discharge arc is formed between the electrodes cathode 5 and anode 4. In this example, the anode 4 is the high-temperature component 1 and is provided with a coating 3 according to the invention. The coating 3 enables the anode 4 to emit a higher thermal radiation output, thereby reducing its temperature and increasing its service life. Similarly, the cathode 5, or both the anode 4 and the cathode 5, may be provided with the coating 3. Naturally, the coating 3 according to the invention may also be used in other types of lamps to increase their thermal emissivity.

[0100] 6 shows a heating conductor 7 made of a refractory metal in an exemplary configuration as a bottom heater of a high-temperature furnace. The heating conductor 7 is heated by direct current and heats the interior of the high-temperature furnace by radiating heat.

[0101] The heating conductor 7 forms in this example the high-temperature component 1 and is provided with a coating 3 according to the invention for increasing the thermal emissivity.

[0102] The coating 3, when applied onto the heating conductor 7, has the effect of producing a given heating output at a lower temperature, thereby reducing creep of the heating conductor 7 and increasing its service life.

[0103] FIG. 7 shows a schematic diagram of a crucible 8 made of a heat-resistant metal. Heat-resistant crucibles are used, for example, to melt aluminum oxide in the production of sapphire single crystals. To do this, the crucible is placed in a high-temperature furnace, where it is heated by a heating conductor via radiant heat. Heat transfer occurs primarily through the sides of the crucible, which absorb the radiant heat and transfer it to the object to be melted. In this example, the crucible 8 forms the high-temperature part 1 and is provided with a coating 3 according to the present invention for increasing thermal emissivity. When applied to the crucible 8, the coating 3 has the effect that a greater proportion of the heat emitted by the heating conductor is coupled into the crucible 8. This allows the crucible 8 to react more quickly to the heat input from the heating conductor.

[0104] The applications of coating 3 are in no way limited to the examples shown here. Coating 3 is generally advantageous for high temperature components where heat transfer is by radiation. [Explanation of symbols]

[0105] 1. High-temperature parts 2. High-temperature component substrate 3. Coatings to increase thermal emissivity 4 anodes 5 cathode 6. High-pressure discharge lamp 7 Heating conductor 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, said coating (3) consisting of at least 55% by weight of rhenium and at least 10% by weight of tungsten, with at least 35% by weight of Re. ３ It has a W phase, The coating (3) has a thermal emissivity of ε>0.6 at room temperature in the wavelength range of 1700-2500 nm.

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

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

4. The high-temperature component (1) according to any one of claims 1 to 3, wherein the coating (3) is formed as a sintered layer.

5. The high-temperature component (1) according to any one of claims 1 to 3, wherein the coating (3) is formed as a PVD layer.

6. The high-temperature component (1) according to any one of claims 1 to 5, wherein the coating (3) is formed on the surface of the high-temperature component (1).

7. The hot component (1) according to any one of the preceding claims, characterized in that the hot component (1) is configured as an electrode (4, 5) of a high-pressure discharge lamp (6).

8. The high-temperature component (1) according to any one of claims 1 to 6, wherein the high-temperature component (1) is formed as a heating conductor (7).

9. The high-temperature component (1) according to any one of claims 1 to 6, wherein the high-temperature component (1) is formed as a crucible (8).

10. A method for manufacturing a high-temperature component (1) having a coating (3) for increasing thermal emissivity, comprising the following steps: - providing a substrate (2) for said high-temperature component (1), followed by step i), step ii) or step iii) below. Step i) - a target material containing tungsten and rhenium, with at least 35% by weight of Re ３ coating said substrate (2) with tungsten and rhenium by physical vapor deposition using a target material having a W phase; Step ii): A step consisting of the following substeps: - coating said substrate (2) with tungsten and rhenium by physical vapor deposition using a target material containing tungsten and rhenium; The coated substrate (2) is heat-treated at a heat treatment temperature of 500° C. or higher in an inert or reducing atmosphere or under high vacuum to obtain a coating of at least 35% by weight of Re. ３ forming a W phase; and - cooling the substrate (2) to room temperature at a cooling rate of more than 20 K / min to remove the at least 35% by weight of Re ３ a sub-step of stabilizing the W phase; Step iii): A step consisting of the following sub-steps: - coating said substrate (2) by powder metallurgy with a powder mixture containing rhenium and tungsten in a molar ratio of 25 atomic % tungsten to 75 atomic % rhenium; The coated substrate (2) is heat-treated at a heat treatment temperature of 500° C. or higher in an inert or reducing atmosphere or under high vacuum to obtain a coating of at least 35% by weight of Re. ３ forming a W phase; and - cooling the substrate (2) to room temperature at a cooling rate of more than 20 K / min to remove at least 35% by weight of Re ３ a sub-step of stabilizing the W phase; and - providing on said substrate (2) said coating (3) having a thermal emissivity ε>0.6 at room temperature for the wavelength range 1700-2500 nm, A method comprising:

11. 11. The method according to claim 10, wherein in step i) or step ii) the substrate (2) is subjected to an enlargement of its surface area before coating the substrate (2).

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

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

14. 11. The method according to claim 10, wherein in step iii) the coating of the substrate (2) is carried out by a slurry method.

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