Tool made of refractory metal

Abstract

A tool (1) made of refractory metal and having a cooling bore (2), wherein a silver-based protective coating (3) is formed at least in some areas of a wall (3) of the cooling bore (2).

Description

[0001] REFRACTORY METAL TOOL

[0002] The present invention relates to a tool made of refractory metal having the features of the preamble of claim 1.

[0003] In the context of this application, refractory metals are understood to mean the metals of Group 4 (titanium, zirconium, and hafnium), Group 5 (vanadium, niobium, tantalum), and Group 6 (chromium, molybdenum, tungsten) of the Periodic Table, as well as rhenium and alloys of the aforementioned elements (refractory metal alloys). Refractory metal alloys are alloys containing at least 50 at. % (atomic percent) of the relevant metal.

[0004] Elements are meant. For clarification and to avoid translation errors, it should be noted that ceramic refractory material is referred to as "refractory / refractories" in English. In German, "refractory" refers to a specific group of metals.

[0005] Tools made of refractory metal are used in industry particularly when particularly high temperature resistance and / or corrosion resistance is required.

[0006] Examples of refractory metal tools include, but are not limited to:

[0007] Piercing plugs for the production of seamless tubes, substrate supports and susceptors for crystal growth, nozzle tips for hot runner nozzles, and glass melt components. In the context of this application, glass melt components are understood to be components intended for use in contact with glass melts. In particular, the invention relates to a glass melt component in the form of a glass melt electrode.

[0008] In applications where protection of the refractory metal tool from oxidative attack by a continuous inert gas atmosphere or similar is not possible, the refractory metal tool must generally be cooled. This is especially true for molybdenum-based tools, which are particularly resistant to thermal stress and many corrosive media. Unprotected molybdenum, on the other hand, is not resistant to air. High-temperature corrosion in air begins at approximately 300°C, with the corrosion rate increasing with increasing temperature.

[0009] A problem with cooling refractory metal tools, especially when using aqueous cooling media, is corrosive attack of the refractory metal at the cooling device, such as a cooling bore. Cooling water can evaporate and corrode the refractory metal tool from the inside. This phenomenon of hot steam corrosion hollows out the tool from within, leading to the failure of the entire component.

[0010] The object of the invention is to provide an improved tool made of refractory metal.

[0011] This object is achieved by a tool having the features of claim 1. Preferred developments are specified in the dependent claims. Furthermore, a method for producing the tool is specified.

[0012] The invention relates to tools made of refractory metal that have a device for supplying a cooling medium. The device for supplying a cooling medium is, in particular, a cooling bore.

[0013] In a broader sense, in the context of this application, a recess intended for the application of a cooling medium can also be referred to as a cooling bore.

[0014] In particular, the cooling hole is a cavity. This can, for example, run through the tool as a channel or be designed as a blind hole.

[0015] By having a cooling bore, with a silver-based protective coating applied at least partially to a wall of the cooling bore, corrosive attack by a cooling medium is prevented. The wall of the cooling bore is effectively protected from corrosive attack by the protective coating. "Silver-based" in the context of this application means that the dominant component of the protective coating is silver. In particular, it means that the composition of the protective coating is greater than or equal to 50 wt.% (percent by weight) silver.

[0016] The resistance of the silver-based protective coating to the cooling medium water has proven to be particularly good in tests carried out by the applicant.

[0017] The advantages of a protective coating based on silver (Ag) are particularly its good thermal and electrical conductivity, good alloyability and an acceptable price.

[0018] More preferably, the protective coating has a composition with greater than or equal to 60 wt.% silver.

[0019] More preferably, the protective coating has a composition with a silver content between 50 wt.% - 90 wt.% silver.

[0020] The protective coating preferably has a composition with a silver content greater than or equal to 50 wt.% and a copper content greater than or equal to 20 wt.%. Additionally, the protective coating may contain other alloying elements such as palladium and / or indium.

[0021] More preferably, the protective coating has a composition with a silver content of between 50 wt.% - 70 wt.% silver, 20 wt.% - 30 wt.% copper with optionally further alloying elements.

[0022] For the compositions, the components, including unavoidable impurities, add up to 100 wt.%. Unavoidable impurities are typically present in a content of less than or equal to 1 wt.%, in particular less than or equal to 0.5 wt.%. An economic advantage of using silver or silver alloys is that they are commercially available as solder alloys. Therefore, silver-based solder alloys are also used to form the protective coating according to the invention.

[0023] The applicant has determined that silver-based solder alloys are also suitable for creating a protective coating. In addition to their chemical suitability as a protective coating, silver-based solder alloys also exhibit good wetting properties, allowing for the creation of well-adhering protective coatings.

[0024] In addition, silver-based solder alloys generally exhibit high strength. A protective coating formed from them exhibits favorable mechanical properties, particularly high strength.

[0025] Particularly preferred is the solder alloy known as "SCP1," with a composition of 68.4 wt.% silver, 26.6 wt.% copper, and 5 wt.% palladium. This solder alloy exhibits very good wetting properties compared to molybdenum.

[0026] Preferably, the protective coating has a thickness between 0.2 pm and 1500 pm.

[0027] More preferably, the protective coating has a thickness between 0.5 pm and 1000 pm.

[0028] Preferably, a mediator layer is provided on the wall of the cooling bore, on which the silver-based protective coating is formed. In other words, the silver-based protective coating does not have to be formed directly on the substrate, i.e., on the wall of the cooling bore. Rather, a mediator layer can be present between the substrate and the protective coating.

[0029] The intermediary layer is preferably metallic, particularly preferably made of copper or a copper alloy, or nickel or a nickel alloy. Copper and nickel can be readily deposited on refractory metals, especially molybdenum. The intermediary layer supports the bonding of the protective coating to the refractory metal substrate.

[0030] The intermediary layer preferably has a thickness of a few microns. In particular, the intermediary layer has a thickness between 0.5 microns and 2 microns.

[0031] The presence of a mediator layer is particularly advantageous for chemical or electrochemical deposition of the protective coating. The mediator layer can support uniform deposition of the protective coating.

[0032] The provision of an intermediary layer can also be advantageous for a protective coating deposited from a melt.

[0033] In particular, an intermediary layer brings about an equalization of the thermal expansion coefficients of the refractory metal substrate and the protective coating.

[0034] The protective coating may have a microstructure with a solidification structure. In other words, the protective coating may be formed from a solidified metal melt.

[0035] A person skilled in the art can easily distinguish a layer produced from a metal melt from a layer deposited alternatively, for example electrochemically.

[0036] This protective coating can be achieved, for example, by melting a suitable amount of a metal composition for the subsequent protective coating in the cooling bore of the tool. In particular, this development proposes forming the protective coating from a solder alloy.

[0037] In an alternative design, the protective coating has a columnar microstructure. This microstructure with columnar crystallites is particularly dense and robust.

[0038] The protective coating can be achieved in particular by electrochemically depositing it. Preferably, the tool is formed, at least in sections, from molybdenum or a molybdenum alloy, for example, TZM.

[0039] Silver and silver compounds exhibit good wetting properties with molybdenum. The resulting protective coating has particularly good adhesion.

[0040] In particular, the tool is obtained via a powder metallurgical manufacturing route. A powder metallurgical manufacturing route of the tool comprises providing a powder comprising a

[0041] Refractory metal, powder consolidation, for example, through pressing, and sintering. Advantages of the powder metallurgy production route include lower energy consumption compared to a smelting metallurgy route and a uniform microstructure.

[0042] Any residual porosity that is usually present in the structure of a powder metallurgically obtained tool is advantageously closed by the protective coating.

[0043] Thus, the protective coating according to the invention is particularly advantageous in connection with a powder metallurgically produced tool.

[0044] The cooling hole can be created through machining or through primary forming. This also includes the creation of a cooling channel through additive manufacturing.

[0045] It may be intended that the tool is obtained by additive manufacturing.

[0046] The tool is preferably designed as a glass melting electrode.

[0047] Glass melting electrodes are used for the electrical heating of glass melts. In particular, the glass melt component is a so-called top electrode.

[0048] These special glass melting electrodes are attached to the edge of a glass melting tank with arms and extend towards the center of the tank.

[0049] There they penetrate the batch blanket and are partly covered with molten glass, but partly in air.

[0050] Top electrodes, a special type of glass melting electrode, are subject to particular stresses: since they are not completely surrounded by the molten glass, they are subject to oxidative attack from the surrounding atmosphere. Furthermore, there is a strong temperature gradient along the length of the electrodes. The invention is particularly beneficial for top electrodes, a special type of glass melting electrode, because, due to their design, part of the electrode is exposed to the atmosphere during operation. This requires cooling of the electrode.

[0051] Molybdenum electrodes would oxidize severely in air and in the mixture layer, leading to sublimation of molybdenum oxide. To prevent this, the top glass melt electrodes are cooled internally with cooling water.

[0052] By ensuring a maximum temperature of approximately 200-300 °C (degrees Celsius) in the area where the electrodes are exposed to the atmosphere, oxidation can be successfully suppressed.

[0053] If the flow of the cooling medium inside the electrode is suboptimal, localized buildup at the tip of the hole can occur, leading to overheating. The cooling water can evaporate and corrode the electrode from within. This phenomenon of superheated steam corrosion hollows out the electrode from within until the entire electrode fails. The protective coating on the cooling hole effectively prevents corrosive attack at the cooling hole.

[0054] In particular, it is provided that the glass melting electrode has a holding section and the cooling bore in the glass melting electrode extends from the holding section in a length between 10% and 70% of a total length of the glass melting electrode.

[0055] More preferably, the cooling bore extends along a length between 20% and 50% of the total length of the glass melting electrode.

[0056] This refinement describes the case where, in particular, only the section of the glass melting electrode located outside the molten bath is to be cooled. This is because, during use, as little energy as possible is to be removed from the glass melt by cooling the glass melting electrode. Rather, the aim is to ensure that essentially only the section of the glass melting electrode that is exposed to oxidative attack by the atmosphere and the cooling medium itself is cooled.

[0057] Particularly preferably, the glass melting electrode is made of molybdenum or a molybdenum alloy and is manufactured using powder metallurgy.

[0058] According to a further embodiment, the tool is designed as a piercing mandrel.

[0059] Manufacturing examples

[0060] The following routes are particularly suggested for the formation of the protective coating:

[0061] 1) Electrochemical deposition of a precious metal layer from a solution. The advantage of electrochemical deposition is the good adjustability of the protective coating, since the fill level of the solution in the bore determines the coating area. Deposition can take place at room temperature. No protective gas / acuum is required during deposition. Multi-layer coatings can be produced, and the layer thickness can be precisely adjusted by selecting the parameters voltage, current, and solution concentration.

[0062] In a first manufacturing example of electrochemical deposition of silver from an aqueous solution, a silver nitrate solution was introduced into the deep hole. The fill level determined the resulting coating thickness. A silver wire was inserted into the hole in a plastic tube. The tube was designed to prevent direct contact with the molybdenum and thus prevent a short circuit. Applying a voltage, with the molybdenum component serving as the cathode and the Ag wire as the anode, resulted in a reduction of the Ag+ ions on the molybdenum surface.

[0063] The rate of silver deposition can be adjusted depending on the concentration of the silver nitrate solution and the applied voltage / current. Intermediate layers of Cu, Ni, or other metals are conceivable as adhesion promoters between Mo and Ag. A copper intermediary layer has proven particularly effective. Annealing after the deposition of a thin Ag layer, followed by a second coating step, could further improve adhesion.

[0064] The manufacturing route described above can be flexibly adapted to the specific coating requirements. By choosing a different electrolyte and anode material, other metallic coatings can also be realized.

[0065] Parameters such as the height of the coating and the thickness of the coating can be advantageously adjusted.

[0066] 2) Melting of precious metal solder in the cooling hole

[0067] The advantages of this route are particularly the good wetting by a silver-based precious metal solder and the particularly easy production of the protective coating.

[0068] In a manufacturing example following this route, after the cooling hole was produced as a deep hole, the inner surface of the cooling hole was thoroughly cleaned.

[0069] A defined amount of silver solder was introduced into the cooling hole. To ensure good wetting of the walls, the solder wire was wound into a coil. The electrode, containing the solder, was then placed in a protective gas atmosphere to prevent oxidation and heated.

[0070] The melting point of the solder used was between 805 and 820 °C. The solder melted in the hole by heating it from the outside with a welding torch. Care was taken to ensure that the melting temperature was not exceeded by too much. After cooling, the electrode can be removed and is ready for use.

[0071] An intermediary layer between the substrate, i.e. the surface of the tool to be coated, and the protective coating can be provided.

[0072] The manufacturing route by melting precious metal solder in the cooling hole is particularly suitable for cooling holes with a small aspect ratio (ratio of the length to diameter of the cooling hole). In particular, the aspect ratio is less than or equal to 10. To give a numerical example, with a cooling hole length of 5 cm and a diameter of 0.5 cm, the aspect ratio would be 10.

[0073] 3) Chemical deposition from a solution

[0074] An example of this route is the Liebig silver deposition process, in which a silver nitrate solution is mixed with an ammonia solution and ammonium sulfate. The process and other agents are familiar to those skilled in the art. A silver mirror deposits from the solution onto the substrate. Unlike route 1), the reaction proceeds without the application of a power source.

[0075] The manufacturing process of the protective coating is particularly simple.

[0076] No electrolyte exchange in the cooling bore is necessary.

[0077] It is particularly advantageous – for all three described routes for forming the protective coating – to provide a mediator layer on the inner wall of the bore. This mediator layer can be formed, for example, based on copper and / or nickel.

[0078] The invention is explained in more detail below with reference to the attached figures.

[0079] From the figures show:

[0080] Fig. 1 a schematic representation of a glass tank with glass melting electrodes as an example of a tool made of refractory metal

[0081] Fig. 2a, 2b a representation of a glass melting electrode

[0082] Fig. 3 a piercing mandrel as an embodiment of the tool

[0083] Fig. 4 the piercing mandrel in cross section

[0084] Fig. 5 shows a substrate carrier as an exemplary embodiment of the tool. Fig. 6 shows a schematic representation of a tool with a protective coating. Figure 1 shows a schematic representation of a glass tank - W. In the exemplary embodiment shown here, the tool 1 is made of refractory metal and is designed as a glass melting electrode 11. An arrangement with two glass melting electrodes 11 is shown.

[0085] In the glass tank W there is a glass melt M which is covered by a batch blanket 5.

[0086] The glass melting electrodes 11 protrude through the batch blanket 5 into the glass melt M.

[0087] The glass melting electrodes 11 are each held on a holding section 9 via electrode holders 7 by cantilevers 8. The glass melting electrodes 11 are supplied with power and cooling medium via the cantilevers 8.

[0088] A cooling bore 2 formed in the glass melting electrodes 11 extends from the holding section 9 to a level of the glass melt - M - .

[0089] Typically, the arrangement is such that the cooling bore 2 ensures cooling of the holding section 9 during normal variations in the level of the molten glass - M - during operation of the glass tank W. A protective coating 4 based on silver is formed on a wall 3 of the cooling bore 2. The protective coating 4 protects the wall 3 from corrosive attack, in particular hot steam corrosion, caused by evaporating coolant.

[0090] Dimensions of the arrangement are, for example:

[0091] Thickness of the mixture layer 5: approx. 200 mm, depth of the cooling hole 2: approx. 350 mm.

[0092] The glass melting electrode 11 is preferably made of molybdenum or an alloy comprising molybdenum. In particular, the glass melting electrode 11 is manufactured using powder metallurgy.

[0093] Figure 2a shows a schematic representation of a glass melting electrode 11 as a design of the tool 1. The glass melting electrode 11 has a cooling bore 2, which is designed here as a blind hole.

[0094] A coolant supply line 6, designed as a coolant lance, extends into the cooling bore 2 via an electrode holder 7. The coolant return line is not shown in the schematic illustration. A silver-based protective coating 4 is applied to a wall 3 of the cooling bore 2.

[0095] In this example, the cooling hole 2 extends over approximately 50% of the length of the glass melting electrode 11.

[0096] Figure 2b shows the conditions of Figure 2a in detail. The flow conditions of a coolant K introduced from the coolant supply line 6 are indicated by arrows.

[0097] Figure 3 shows a further embodiment of the invention, in which the tool 1 is designed as a piercing mandrel 12.

[0098] Figure 4 shows the piercing mandrel 12 of Figure 3 in cross section.

[0099] The piercing mandrel 12 has a cooling bore 2, on the wall 3 of which a protective coating 4 based on silver is formed.

[0100] During operation, the piercing mandrel 12 can be cooled by coolant (not shown) introduced into the cooling bore 2. The protective coating 4 protects the wall 3 from corrosive attack, particularly hot steam corrosion, caused by evaporating coolant.

[0101] The piercing mandrel 12 is preferably made of molybdenum or an alloy comprising molybdenum. In particular, the piercing mandrel 12 is manufactured using powder metallurgy.

[0102] Figure 5 schematically shows a further embodiment of the invention, in which the tool 1 is designed as a substrate carrier 13 for depositions from the gas phase.

[0103] A substrate carrier 13 serves as a support surface for a substrate to be evaporated or deposited. Substrate carriers 13 are particularly used in the deposition of layers.

[0104] In particular, the substrate carrier 13 is designed for the growth of synthetic diamonds. A seed crystal, on which the deposition takes place, can be placed on a surface of the substrate carrier 13.

[0105] For cooling the substrate carrier 13, a cooling bore 2 is provided, on whose wall 3 a protective coating 4 based on silver is formed. The protective coating 4 protects the wall 3 from corrosive attack, in particular hot steam corrosion, caused by evaporating coolant.

[0106] The substrate carrier 13 is preferably made of molybdenum or an alloy comprising molybdenum. In particular, the substrate carrier 13 is manufactured using powder metallurgy.

[0107] Figure 6 schematically shows a tool 1 with a protective coating 4. In the embodiment shown here, the protective coating 4 is not formed directly on the wall 3, but rather a mediator layer 41 is present on the wall 3. It may be helpful to first

[0108] To deposit the intermediary layer 41 onto the refractory metal substrate and only then to form the protective coating 4.

[0109] The protective coating 4 can therefore be formed directly on the wall 3 or indirectly, coupled to the wall 3 via an intermediary layer 41.

Claims

Claims 1 . Tool (1) made of refractory metal with a cooling bore (2), wherein a protective coating (4) based on silver is formed at least in sections on a wall (3) of the cooling bore (2).

2. Tool (1) according to claim 1, wherein the protective coating (4) has a silver content of greater than or equal to 50 wt.%.

3. Tool (1) according to claim 1 or 2, wherein the protective coating (4) has a silver content of greater than or equal to 50 wt.% and a copper content of greater than or equal to 20 wt.%.

4. Tool (1) according to one of the preceding claims, wherein the protective coating (4) has a thickness between 0.2 pm and 1500 pm.

5. Tool (1) according to one of the preceding claims, wherein on the wall (3) there is at least in sections an intermediary layer (41) on which the protective coating (4) is formed.

6. Tool (1) according to one of the preceding claims, wherein the tool (1) is made of molybdenum or a molybdenum alloy.

7. Tool (1) according to one of the preceding claims, wherein the tool (1) is designed as a glass melting electrode (11).

8. Tool (1) according to one of the preceding claims 1 to 6, wherein the tool (1) is designed as a piercing mandrel (12).

9. Tool (1) according to one of the preceding claims 1 to 6, wherein the tool (1) is designed as a substrate carrier (13).

10. Method for producing a tool (1) with a cooling bore (2), wherein on a wall (3) of the cooling bore (2) a deposition of a protective coating (4) based on silver is carried out by - chemical or electrochemical deposition of silver or a composition containing silver or - by melting silver or a silver alloy in the cooling bore (2), wherein optionally a mediator layer (41) is previously deposited on the wall (3) for bonding the protective coating (4).

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