HIGH-TEMPERATURE COMPONENT

Abstract

A high-temperature component made of a refractory metal or a refractory metal alloy, includes a coating for increasing thermal emissivity. The coating is formed substantially of tungsten and rhenium, i.e. of at least 55 wt. % rhenium and at least 10 wt. % tungsten, and has a Re3W phase of at least 35 wt. %. A process for producing a high-temperature component having a coating for increasing thermal emissivity, is also provided.

Claims

1-14. (canceled)

15. A high-temperature component based on refractory metal or a refractory metal alloy, the high-temperature component comprising: a coating for increasing thermal emissivity, said coating consisting essentially of tungsten and rhenium including at least 55% by weight of rhenium and at least 10% by weight of tungsten, and said coating having a Re.sub.3W phase of at least 35% by weight.

16. The high-temperature component according to claim 15, wherein said coating is porous.

17. The high-temperature component according to claim 15, which further comprises a structured surface of the high-temperature component below said coating.

18. The high-temperature component according to claim 15, wherein said coating is formed as a sintered layer.

19. The high-temperature component according to claim 15, wherein said coating is formed as a PVD layer.

20. The high-temperature component according to claim 15, which further comprises a top side of the high-temperature component, said coating being formed on said top side.

21. The high-temperature component according to claim 15, wherein the high-temperature component is formed as an electrode of a high-pressure discharge lamp.

22. The high-temperature component according to claim 15, wherein the high-temperature component is formed as a heating conductor.

23. The high-temperature component according to claim 15, wherein the high-temperature component is formed as a crucible.

24. A process for producing a high-temperature component having a coating for increasing thermal emissivity, the process comprising steps of: providing a main body of the high-temperature component; i) coating the main body with tungsten and rhenium by physical vapor deposition using a target material including tungsten and rhenium, the target material including at least 35% by weight Re.sub.3W phase; or ii) coating the main body with tungsten and rhenium by physical vapor deposition using a target material including tungsten and rhenium, heat treating the coated main body at a heat treatment temperature of at least 500° C. in an inert or reducing atmosphere or high vacuum to form the Re.sub.3W phase, and cooling the main body to room temperature at a cooling rate of greater than 20 K/min to stabilize the Re.sub.3W phase; or iii) coating the main body with a powder mixture containing rhenium and tungsten with a molar ratio of 25 at. % tungsten to 75 at. % rhenium by a powder metallurgical process, heat treating the coated main body at a heat treatment temperature of at least 500° C. in an inert or reducing atmosphere or high vacuum to form the Re.sub.3W phase, and cooling the main body to room temperature at a cooling rate of greater than 20 K/min to stabilize the Re.sub.3W phase.

25. The process according to claim 24, which further comprises enlarging a surface area of the main body before coating the main body in step i) or ii).

26. The process according to claim 25, which further comprises enlarging a surface area of the main body of the high-temperature component in step i) or ii) by providing a slurry coating of the main body.

27. The process according to claim 25, which further comprises enlarging a surface area of the main body of the high-temperature component in step i) or ii) by mechanical, chemical or thermal structuring of the main body.

28. The process according to claim 24, which further comprises coating the main body by using a slurry process in step iii).

Description

[0085] The invention is elucidated in more detail below with reference to following production examples and drawings. Shown is:

[0086] FIG. 1: a phase diagram of the binary tungsten-rhenium system,

[0087] FIG. 2a-2d: scanning electron micrographs of surfaces coated according to the invention in cross section (fracture surfaces) (FIGS. 2a and 2c) and in top view (FIGS. 2b and 2d),

[0088] FIG. 3: a diagram with values for the thermal emissivity epsilon (c) for various coatings,

[0089] FIG. 4a, 4b: X-ray diffractograms (XRD) of a layer produced according to the invention and a conventionally produced layer,

[0090] FIG. 5: schematically a high-pressure discharge lamp as an exemplary embodiment of a high-temperature component,

[0091] FIG. 6: a heating conductor as an exemplary embodiment of a high-temperature component,

[0092] FIG. 7: a crucible as an exemplary embodiment of a high-temperature component.

PRODUCTION EXAMPLE I

[0093] To produce the high-temperature component according to production example I, main bodies composed of tungsten were coated with slurries of different powder mixtures. For this purpose, first tungsten powder and/or rhenium powder were weighed out into a binder of 2% by weight ethyl cellulose in ethanol to a total solids content of 50%. Stirring took place at 1500 rpm for 15 minutes with a Multimaster apparatus from Netzsch.

[0094] Samples were prepared for the following layer compositions: [0095] 100% by weight tungsten [0096] 10% by weight rhenium, remainder tungsten [0097] 20% by weight rhenium, remainder tungsten [0098] 30% by weight rhenium, remainder tungsten [0099] 40% by weight rhenium, remainder tungsten [0100] 50% by weight rhenium, remainder tungsten [0101] 60% by weight rhenium, remainder tungsten [0102] 70% by weight rhenium, remainder tungsten [0103] 80% by weight rhenium, remainder tungsten [0104] 90% by weight rhenium, remainder tungsten [0105] 100% by weight rhenium.

[0106] The weight percentages given here refer to the weight of the solid components rhenium and tungsten and also correspond to the weight percentages in the layer, since the organic constituents volatilize during the heat treatment.

[0107] Spray coating was then effected manually on tungsten platelets at a distance of ca. 20 cm so as to give a target layer mass of 15 mg/cm.sup.2. Drying took place in ambient air.

[0108] The dried-on layer was then subjected to a heat treatment (annealing). Organic components (e.g. binders) volatilize as a result of the heat treatment and the layer is consolidated. Each heat treatment was conducted at 1800° C. for 20 hours under an argon (Ar) atmosphere. After the heat treatment, the coated main body is slowly cooled stepwise over a period of 10 hours to 800° C. (corresponding to an average cooling rate of 1.67 K/min) and quenched from ca. 800° C. within 20 minutes to room temperature (corresponding to an average cooling rate of around 40 K/min).

[0109] For comparison purposes, additional samples of 80% by weight rhenium, remainder tungsten, were produced analogously to the production process described previously, except that they were heat treated for 6 hours at 1600° C. under an argon atmosphere rather than for 20 hours.

[0110] The thermal emissivity of the layers was measured using a Solar 410 Reflectometer from Surface Optics Corporation at room temperature and at a wavelength range between 1700-2500 nm, since this infrared wavelength range is particularly relevant for assessing the thermal radiation of a body.

[0111] In the tables below, the measurement results are also contrasted with known values of the thermal emissivity of coatings known from the prior art, for example coatings with tantalum nitride according to WO2018204943 (A2).

[0112] A selection of results is summarized in Table 1, a more detailed representation is given in the graph of FIG. 1, where the thermal emissivity epsilon (c) is shown as a function of the rhenium content.

TABLE-US-00001 TABLE 1 Comparison of thermal emissivity for different coatings Thermal Sample emissivity ε Observation 1 Tungsten slurry 0.34 Prior art 2 Rhenium slurry 0.36 Prior art 3 Tantalum nitride slurry 0.89 Prior art (WO2018204943), T.sub.max = 1500° C. 4 Rhenium (80%) + 0.35 Sintered only 6 tungsten (20%) hours 5 Rhenium (80%) + 0.66 20 h sintered tungsten (20%)

[0113] Sample No. 1, a porous tungsten coating obtained with 100% tungsten slurry has a thermal emissivity of 0.34. Sample No. 2, a porous rhenium coating obtained with 100% rhenium slurry has a thermal emissivity of 0.36. Sample No. 3 is a coating of tantalum nitride that was produced in accordance with the details in WO2018204943 of the applicant. This has a comparatively high thermal emissivity of 0.89, but it can only be used for temperatures up to a maximum of 1500° C. Sample No. 4 has a coating composed of 80% rhenium, 20% tungsten, which has been prepared as described above for comparison purposes with a heat treatment at 1600° C. for 6 hours. As explained in more detail below, this sample has primarily tungsten/rhenium solid solutions and only a very small fraction of Re.sub.3W phase. It has a thermal emissivity of 0.35. Sample No. 5 is an 80% rhenium and 20% tungsten coating prepared according to the instructions previously described (heat treatment at 1800° C. for 20 hours). The proportion of Re.sub.3W phase is ca. 90% by weight. The thermal emissivity was determined to be 0.66.

[0114] FIGS. 2a to 2d show scanning electron micrographs of Sample No. 5. FIGS. 2a and 2b are an image magnified 1000-fold, and FIGS. 2c and 2d show an image magnified 3000-fold. FIGS. 2a and 2c show a fracture surface normal to the surface of the sample, FIGS. 2b and 2d are a top view of the surface, i.e. the viewing direction is normal to the coated surface. In the fracture surfaces, the substrate 2 composed of tungsten sheet material can be seen in the lower part of the figure. The porous coating 3 can be seen above it. The porosity increases the microscopic surface area and contributes to a further increase in thermal emissivity.

[0115] FIG. 3 shows, in a graph, the measured thermal emissivities epsilon (ε) for the test series mentioned at the outset having different rhenium contents. The rhenium content is plotted on the abscissa and the measured thermal emissivity epsilon (c) is plotted on the ordinate. The points in the diagram denote the respective measurement values. The dashed line Eth (epsilon theoretical) marks the thermal emissivity values that would be expected when linearly interpolating the thermal emissivity from 100% by weight tungsten to 100% by weight rhenium. It can be seen that, particularly in the range between 50% by weight and 90% by weight rhenium, the measured values for the thermal emissivity surprisingly do not run along this straight line Eth but above it, sometimes very significantly above. There is a maximum value for the thermal emissivity in the range between 70 and 80% by weight rhenium. Measurements by the applicant indicate that the advantageous increase in the thermal emission value is likely to be due to the presence of the Re.sub.3W phase.

[0116] This is illustrated in Table 2. Table 2 shows the results of a detailed quantitative phase analysis for samples having a rhenium content of 70% by weight (sample I) and 80% by weight (sample II). For the quantitative determination of the phases, part of the coating of the respective sample was scraped off, ground to a powder and analyzed by XRD. For comparison, measured values for samples (sample Ia and sample IIa) that were produced in a conventional way (i.e. with a heat treatment duration of 6 hours) are also given.

TABLE-US-00002 TABLE 2 Phase analysis Phase Phase Phase Phase Thermal Sample W.sub.0.5Re.sub.0.5 Re.sub.3W (Re) (W) emissivity ε I Re 70 wt. %, 10.5 89.5 0.64 W 30 wt. % Ia Re 70 wt. %, 59.9 21.8 18.3 0.35 W 30 wt. % (6 h) II Re 80 wt. %, 90.4 9.6 0.66 W 20 wt. % IIa Re 80 wt. %, 57.1 27.8 15.1 0.35 W 20 wt. % (6 h)

[0117] (W) and (Re) are both solid solution phases ((W) is a tungsten crystal with rhenium dissolved therein, analogously (Re) is a rhenium crystal with tungsten dissolved therein). W.sub.0.5Re.sub.0.5 is an intermetallic phase and is also shown in the phase diagram as σ-phase. The quantities in the phases are in % by weight.

[0118] The measurement results show that in the samples with the coating according to the invention, which have been heat-treated for a significantly longer time, the proportion of Re.sub.3W is significantly higher than in the samples produced with a heat treatment time typically used in powder metallurgical processing of tungsten and rhenium. The portion of the Re.sub.3W is around 90% by weight for both samples, sample I (70% by weight rhenium) and sample II (80% by weight rhenium), whereas the proportion of Re.sub.3W in the corresponding conventionally produced samples is 21.8% by weight (sample Ia) or 27.8% by weight (sample IIa). The high proportion of Re.sub.3W is also associated with a significantly higher thermal emission coefficient.

[0119] FIGS. 4a and 4b show X-ray diffractograms (XRD) of sample II (FIG. 4a) and sample IIa (FIG. 4b). In the diffractograms, intensity values are given as a function of the deflection angle 2Theta (range from 30 to 65 2Theta) 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 Re.sub.3W predominates.

[0120] Table 3 demonstrates the temperature resistance of the samples according to the invention. Shown is the measured value for the thermal emissivity as a function of the temperature at which the sample was subjected to a heat stress test. The samples were annealed at this temperature for the period of one hour.

TABLE-US-00003 TABLE 3 Temperature resistance Heat stress test (temperature) Thermal emissivity ε None 0.66 2000 0.65 2200 0.58

[0121] The series of tests showed that the thermal emissivity at T=2000° C. does not decrease significantly (from 0.66, the value for the sample before the heat stress test, to 0.65) and the coating does not degrade. At T=2200° C., a decrease in thermal emissivity of ca. 12% was observed. The material withstands the high thermal stress, but the porous layer begins to sinter somewhat. Nevertheless, high thermal emissivity is maintained even at this high temperature. The coating according to the invention thus withstands exposures of 2000° C. and above and it can therefore be used for heating filaments in MOCVD systems.

PRODUCTION EXAMPLE II

[0122] An alternative variant for producing the coating is based on physical vapor deposition. In the production example, a tungsten platelet was first coated with a conventional 100% tungsten slurry layer. This serves to increase the surface area. A ca. 4 μm thick layer of Re.sub.3W phase was sputtered onto this layer using a target containing ca. 98% Re.sub.3W phase. The resulting layer had approximately 75% by weight rhenium. Since part of the radiation exchange also occurs via parts of the porous tungsten structure that are not covered by the PVD coating, the measured values for the thermal emissivity did not quite reach the values in production example I.

[0123] Application examples for the high-temperature component are explained below with reference to FIGS. 5 to 7.

[0124] In FIG. 5, a high-pressure discharge lamp 6 is shown schematically. A discharge arc is formed between the electrodes—a cathode 5 and an anode 4—during operation. In the present exemplary embodiment, the anode 4 is the high-temperature component 1 and is provided with a coating 3 according to the invention. The coating 3 allows the anode 4 to emit a higher thermal radiation output, which reduces its temperature and increases the service life. Likewise, the cathode 5 or both the anode 4 and the cathode 5 can be provided with the coating 3. Clearly, the coating 3 according to the invention for increasing thermal emissivity can also be used for other types of lamps.

[0125] FIG. 6 shows a heating conductor 7 of a refractory metal in an exemplary arrangement as a base heater of a high-temperature furnace. The heating conductor 7 is heated by passing a current directly through it and warms the interior of the high-temperature furnace by giving off radiant heat.

[0126] In the present exemplary embodiment, the heating conductor 7 forms the high-temperature component 1 and is provided with a coating 3 according to the invention for increasing thermal emissivity. When used on a heating conductor 7, the coating 3 allows said heating conductor 6 to produce a given heating output at a lower temperature. This reduces creep of the heating conductor 7 and extends the lifetime.

[0127] FIG. 7 schematically shows a crucible 8 of refractory metal. Crucibles of refractory metal are used for example to melt aluminum oxide in the production of single-crystal sapphires. For this purpose, the crucibles are placed in a high-temperature furnace and warmed there by radiant heat from heating conductors. The heat transfer predominantly takes place via the lateral surface of the crucible, which absorbs the radiant heat and transmits it to the product to be melted. In the present exemplary embodiment, the crucible 8 forms the high-temperature component 1 and is provided with a coating 3 according to the invention for increasing thermal emissivity. When used on a crucible 8, the coating 3 brings about the effect that a greater proportion of the heat given off by heating conductors is coupled into the crucible 8. The crucible 8 thereby reacts more quickly to a heat input from heating conductors.

[0128] The use of the coating 3 is in no way limited to the examples shown here. The coating 3 is generally advantageous for high-temperature components, at which heat transfer by means of radiation is to take place.

LIST OF THE DESIGNATIONS USED

[0129] 1 High-temperature component [0130] 2 Main body of the high-temperature component [0131] 3 Coating to increase thermal emissivity [0132] 4 Anode [0133] 5 Cathode [0134] 6 High-pressure discharge lamp [0135] 7 Heating conductor [0136] 8 Crucible