Molybdenum alloy for use in the temperature range from 500 °c to at least 900 °c

Abstract

The invention relates to a molybdenum alloy for use in the temperature range from 500 °C to at least 900 °C, obtainable via a powder metallurgical process using the starting element molybdenum or a molybdenum-based base alloy and at least one metal oxide as starting materials and a heat treatment at a temperature in the range from 500 °C to a maximum of 1000 °C in an oxidizing atmosphere until a closed metal molybdate layer is formed, wherein the metal of the metal molybdate layer originates from the at least one metal oxide, which is selected from a group consisting of zinc oxide (ZnO), calcium oxide (CaO), a manganese oxide (MnO2, Mn2O3), magnesium oxide (MgO), nickel oxide (NiO). Molybdenum and the at least one metal oxide are mixed with each other in a mass ratio in which molybdenum has the same mass or a mass excess compared to the at least one metal oxide.

Description

[0001] Molybdenum alloy for use in a temperature range of 500 °C to at least 900 °C

[0002] The present invention can be classified within the field of refractory metal-based high-temperature alloys in general and molybdenum alloys in particular. The invention relates to a molybdenum alloy for use in the temperature range of 500 °C to at least 900 °C.

[0003] The efficiency of gas and aircraft turbines could be significantly improved by a slightly higher gas inlet temperature. The nickel-based superalloys currently used in high-temperature applications are severely limited in this respect due to their comparatively low melting point of 1400 °C, meaning that no further significant improvements can be expected from these materials. Molybdenum alloys, which are now very well researched, represent an interesting alternative to nickel-based superalloys in this application area, as described, for example, in Lunk HJ, Hartl H. Discovery, properties and applications of molybdenum and its compounds. ChemTexts 2017; 3. https: / / doi.org / 10.1007 / s40828-017-0048-6.

[0004] German Patent Application GB 2 168723 A discloses a molybdenum-based alloy containing 0.2 to 1 wt% of an oxide of a suitable metal, for example, aluminum, chromium, or zirconium. This alloy is designed for applications at temperatures above 1600 °C and is particularly useful for improving creep resistance at high temperatures in a reducing atmosphere. Such a molybdenum alloy can be produced using a relatively complex, multi-stage process. In a first step, molybdenum and / or molybdenum oxide is mixed with a solution of a salt of the suitable metal. In a second step, the resulting mixture is exposed to a reducing hydrogen atmosphere at a temperature not exceeding 1150 °C. In a further step, the resulting metallic powder is pressed into a shaped object, followed by a sintering step under a hydrogen atmosphere at a temperature in the range of 1750 °C to 2200 °C.

[0005] The greatest challenge with molybdenum alloys is their poor oxidation resistance in the temperature range of 600 °C to 900 °C, which leads to the formation of molybdenum trioxide and consequently to catastrophic material failure. Therefore, these alloys can only be used in this range under vacuum or with a suitable protective gas. To date, the only solutions for improving oxidation resistance involve adding further alloying elements to the molybdenum alloys, such as silicon (Si), boron (B), and titanium (Ti). While these elements are able to form a protective layer at higher temperatures, they cannot completely prevent catastrophic oxidation failure in the aforementioned range.For example, while Mo-Si-B alloys are characterized by both excellent mechanical properties and good oxidation resistance, their oxidation behavior remains a significant disadvantage, as the oxidation mechanism is essentially temperature-dependent. In the temperature range of 600 °C to 900 °C, the oxidation of molybdenum still leads to the formation and volatilization of molybdenum trioxide, which initiates the aforementioned catastrophic material failure. At higher temperatures, i.e., from approximately 1000 °C, a protective borosilicate layer forms after a certain period, protecting the material from further oxidation. At lower temperatures, this protective mechanism does not function due to the high viscosity of the borosilicate layer, making oxidation of the material unavoidable.At even higher temperatures, from approximately 1300 °C, oxidation resistance decreases because boron trioxide (B₂O₃) evaporates under these conditions and consequently from the surface. For molybdenum alloys to be used in technical applications in the future, a significantly more effective oxidation protection strategy is required, one that prevents or at least significantly reduces the formation and / or evaporation of molybdenum trioxide, particularly in the critical temperature range of 600 °C to 900 °C.

[0006] The object of the invention is to provide a molybdenum alloy with improved oxidation resistance in the temperature range from 500 °C to at least 900 °C.

[0007] This problem is solved by a molybdenum alloy according to claim 1, which is suitable for use in a temperature range from 500 °C to at least 900 °C, preferably up to 1000 °C. Further developments are specified in the dependent claims.

[0008] Such a molybdenum alloy is obtainable by a powder metallurgical process using molybdenum or a molybdenum-based alloy as the starting material and at least one metal oxide as starting materials, and by heat treatment at a temperature in the range of 500°C to a maximum of 1000°C in an oxidizing atmosphere, preferably in the presence of atmospheric oxygen, until a continuous metal-molybdate layer is formed, wherein the metal of the metal-molybdate layer originates from the at least one metal oxide selected from the group consisting of zinc oxide (ZnO), calcium oxide (CaO), a manganese oxide (Mn₂O, Mn₂O), magnesium oxide (MgO), and nickel oxide (NiO). Each of the aforementioned oxides is capable of reacting with the volatile molybdenum trioxide and binding it as molybdate.In this process, molybdenum and at least one metal oxide are mixed together in a mass ratio such that molybdenum has the same mass or a mass excess compared to the at least one metal oxide. Preferably, the mass of the at least one metal oxide is at least 30% of the total mass of the starting materials defined as 100% for the powder metallurgy process. Heat treatment is carried out, preferably after drying and compaction of the powder mixture consisting of molybdenum or a molybdenum-based powder with further additives, for example, Mo-B-Si, or a molybdenum-based alloy and at least one metal oxide, preferably zinc oxide. The solids produced in this way are treated in a defined step in an oxidizing atmosphere, for example, in air under static conditions.The samples are typically placed directly into a preheated oven at a temperature of at least 500 °C and up to a maximum of 1000 °C, and removed after a defined holding time, also at this temperature. A holding time of just one hour has proven sufficient to form a continuous, protective molybdate coating on the surface of the samples. This coating is formed by the controlled oxidation of pure molybdenum in the presence of the added metal oxide during the heat treatment. The metal oxide(s) are added in a comparatively high quantity, preferably at least 30% by weight, to enable a complete and effective reaction. The aim of this heat treatment is not to influence mechanical properties, but rather to form an oxidation-resistant protective layer for use at temperatures up to at least 900 °C, preferably up to 1000 °C.This approach differs fundamentally from prior art methods, in which heat treatment takes place in a reducing atmosphere, for example hydrogen, and therefore no molybdate formation is to be expected.

[0009] The invention presented here relates to a novel molybdenum alloy. The addition of a metal oxide, such as zinc oxide, almost completely prevents the volatilization of the molybdenum trioxide formed during molybdenum oxidation, since zinc oxide, or the other metal oxides mentioned above, undergoes a chemical reaction with the volatile molybdenum trioxide. The result is the formation of the respective metal molybdate, which forms a protective coating and thus protects the material from further oxidation.

[0010] This represents a simple and effective method for preventing catastrophic material failure in molybdenum and molybdenum alloys in a temperature range of 500 °C to at least 900 °C, thus enabling these materials to be used in this temperature range without the need for a vacuum or protective gas. Furthermore, depending on the requirements, this opens up possibilities for using molybdenum in applications where its oxidation was previously unfeasible. Another advantage is the ease of producing this alloy. Both starting materials are readily available and inexpensive, and can be processed using powder metallurgy with ease by mixing the two powders in a specific ratio. The resulting powder then only needs to be compacted into a dense solid and subjected to heat treatment.

[0011] More precisely, the novel molybdenum alloys are produced from the starting materials, molybdenum and at least one metal oxide, using powder metallurgy and are subjected to heat treatment until a closed, protective molybdate layer has formed.

[0012] A powder metallurgy process generally comprises (a) powder production, (b) compacting and shaping the powder, usually by pressing, and (c) sintering, the sintering being used to bond and densify the pressed powder by heat treatment.

[0013] Preferably, the mass of molybdenum in the starting materials is 1 to 2.4 times the mass of the at least one metal oxide. A particularly advantageous mixing ratio in the starting materials exists when molybdenum is present in a 1.16 to 1.18-fold mass excess compared to the at least one metal oxide. Molybdate is generally a salt of molybdic acid in which the anion contains both molybdenum and oxygen. Molybdates can be formed, among other ways, by the direct reaction of molybdenum trioxide with various metal oxides. They usually have the general formula MeMoCM₂ (Me = metal) and contain the discrete tetrahedral ion MoCM₂. 2-Molybdates are primarily used for corrosion inhibition, for example in engine coolants and paints, due to their protective effect on ferrous and non-ferrous metals and their extremely low toxicity. Applications of molybdates related to the oxidation protection of molybdenum or molybdenum alloys are not yet known.

[0014] Due to the described chemical "binding" of the volatile molybdenum compounds, the molybdenum alloys are protected from further oxidation and continuous mass loss within the aforementioned temperature range. As a result, a significant improvement in oxidation resistance is achieved in the critical temperature range of 500 °C to at least 900 °C, enabling this material to be used permanently within this temperature range for the first time. This is achieved by adding only one metal oxide capable of reacting with molybdenum trioxide, keeping the system simple and manageable.

[0015] A particularly preferred alloy consists of molybdenum and zinc oxide (Mo-ZnO). These are preferably mixed together in a mass ratio in which molybdenum has the same mass as, or a mass excess of, the zinc oxide, wherein the mass of molybdenum is preferably 1 to 2.4 times the mass of zinc oxide, for example, molybdenum being present in a 1.18-fold excess. Preferably, the mass of the zinc oxide is at least 30% of the total mass of the starting materials in the powder metallurgy process. The addition of zinc oxide almost completely prevents the volatilization of the molybdenum trioxide (MoOs) formed as a result of the oxidation of the molybdenum by forming zinc molybdate, which forms a protective layer and thus protects the material from further oxidation. Zinc molybdate is an inorganic, water-insoluble compound that can exist in two phases.At a temperature of approximately 450 °C, the monoclinic β-phase transforms into the triclinic α-phase. Zinc molybdate is formed by the reaction of zinc oxide with molybdenum trioxide and has a melting point of approximately 1000 °C. Due to its excellent electrical and optical properties, zinc molybdate is used, for example, in catalysis, photoluminescence, and batteries. It is also used as a white pigment and simultaneously serves as a corrosion inhibitor.

[0016] The metal-molybdate layer formed in the molybdenum alloy can consist of one or more zinc-molybdate compounds, for example ZnMoCM and / or Z^MosOs.

[0017] Manganese-molybdate (MnMoCM) is particularly suitable for high-temperature applications due to its relatively high melting point of 1130 °C. Manganese-molybdate possesses excellent electrochemical properties, resulting in high capacitive behavior and long-term stability. For this reason, it is also used as an active electrode material for aqueous supercapacitors and as a catalyst for hydrogen evolution.

[0018] In contrast to zinc-molybdates, the formation mechanism in this case is somewhat more complex, as manganese oxides of varying oxidation states are formed depending on the temperature. The reaction between manganese oxide and molybdenum trioxide occurs at approximately 520 °C, which falls precisely within the temperature range for the conversion of MnO₂ to MnO₃. From this, it can be concluded that MnO₃ likely acts as the reactive intermediate for molybdate formation. Only MnO₃⁴ is not reactive with molybdenum trioxide at temperatures up to 600 °C. The easy formation of manganese-molybdate is due to the microscopic grain size and low crystallinity of the newly formed MnO₂ and MnO₃, which allows the manganese ions to diffuse relatively easily. Manganese(III) oxide (MnO₃) is preferably used as the starting material.

[0019] When nickel oxide is used as a starting material for the powder metallurgical production of the molybdenum alloy described above, nickel molybdate (NiMoCk) is formed. It is known that a phase transition from the α-phase to the β-phase of nickel molybdate occurs at temperatures between 550 °C and 690 °C, with the latter reverting back to the α-phase at approximately 730 °C. Transition metal molybdates, to which nickel molybdate belongs, generally exhibit high redox activity, good electronic conductivity, and a stable crystal structure, which is why they are considered promising electrode materials for energy storage devices.

[0020] When magnesium oxide is used as a starting material for the production of the molybdenum alloy described above, magnesium molybdate (MgMoCk) is formed, which is also known for its good redox reaction properties and suitability for use in supercapacitors. The use of magnesium oxide is particularly advantageous in terms of cost.

[0021] According to a further embodiment of the invention, to improve the protection of the material at high temperatures, one or more additional alloying elements X can be added to the molybdenum alloys as elements or in the form of oxides of these elements, wherein these alloying elements X can be, for example, metals such as aluminum, nickel, titanium, zirconium, vanadium, niobium, tantalum, tungsten, chromium, hafnium, yttrium, lanthanum, and / or metalloids such as boron and / or silicon. As a result of the addition of one or more of these alloying elements to the starting materials of the molybdenum alloy, one or more further phases, each consisting of a compound of one or more of the selected alloying elements with or without molybdenum, can be formed in addition to the metal-molybdate layer.If a mass fraction of at least one metal oxide is defined in the total mass of the starting materials for the powder metallurgy process, the other alloying elements X can only be added in a proportion that maintains the specified composition of the starting materials with respect to the metal oxide content. This means that the metal oxide content in the total mass of the starting materials for the powder metallurgy process is a fixed component, while the other alloy components can vary within the overall composition of the starting materials, which is defined as 100% by weight.

[0022] In the case of the use of silicon as an additional alloying element, further phases such as Mo solid solution, Mo3Si, MosSiB2, MosSis MoSi2 and / or TisSis could be formed. Such alloys can exhibit mixtures with at least one further of the aforementioned alloying elements X at the Mo sites, so that one or more of the following further phases could also result: (Mo,X) solid solution, (Mo,X)3Si, (Mo,X)5SiB2, (Mo,X)5Si3 and (Mo,X)Si2.

[0023] A solid solution (Mk) is a crystal or crystallite consisting of at least two different chemical elements, with the foreign atoms or ions being statistically distributed. These can either be incorporated into the interstitial sites, as in an interstitial solid solution, or replace an atom of the other element through substitution, in a so-called substitutional solid solution. Thus, other elements can be "incorporated" into the crystal lattice of a molybdenum solid solution. These elements have different solubilities within the molybdenum solid solution. For example, molybdenum and vanadium are completely miscible, while the solubility of titanium in molybdenum is only about 12% by atomic fraction (12 at%). Therefore, the aforementioned alloying elements can be dissolved in varying amounts in a molybdenum solid solution without forming new phases or secondary phases.A particularly advantageous embodiment of the molybdenum alloy is achieved by adding the alloying element silicon, either as an element or in the form of silicon dioxide, as a starting material, in addition to molybdenum and zinc oxide. In the powder metallurgy process, existing silicon dioxide or silicon dioxide produced by oxidation can also react with the zinc oxide (ZnO), forming a zinc silicate phase (Zn₂SiÜ₄ or ZnSiO₃) with a melting point of approximately 1500 °C, thus extending the alloy's applicability to high-temperature applications.

[0024] Further details, features, and advantages of embodiments of the invention will become apparent from the following figures describing exemplary embodiments. These show:

[0025] Fig. 1A: a diagram showing the change in the material composition of a Mo-ZnO tablet after one hour of heat treatment,

[0026] Fig. 1 B: a diagram showing the change in the material composition of a Mo-ZnO tablet after 5 hours of heat treatment,

[0027] Fig. 2: a comparative juxtaposition of XRD diffractograms of an untreated, a Mo-ZnO tablet treated at 800 °C and a tablet treated at 900 °C,

[0028] Fig. 3A: an SEM image of a cross-section of a Mo-ZnO sample after 5 hours of heat treatment at 800 °C,

[0029] Fig. 3B: the result of an analysis of a marked area of ​​the cross-section using energy-dispersive X-ray spectroscopy (EDX),

[0030] Fig. 4A: an SEM image of a cross-section of a Mo-ZnO sample after twenty hours of heat treatment at 800 °C, Fig. 4B: the result of an analysis of a labeled area of ​​the cross-section using energy-dispersive X-ray spectroscopy (EDX),

[0031] Fig. 5A: a Raman spectrum of a bright region after 5 hours of heat treatment of the Mn-ZnO sample at 800 °C,

[0032] Fig. 5B: a light microscopic image with a crosshair in which the investigated bright area is located,

[0033] Fig. 6A: a Raman spectrum of a dark region after 5 hours of heat treatment of the Mn-ZnO sample at 800 °C,

[0034] Fig. 6B: a light microscopic image with a crosshair in which the dark area under investigation is located,

[0035] Fig. 7A: Result of the investigation of a bright surface area on the Mo-ZnO tablet by Raman spectroscopy after 5 hours of heat treatment at 700 °C,

[0036] Fig. 7B: Result of the investigation of a dark surface area on the Mo-ZnO tablet by Raman spectroscopy after 5 hours of heat treatment at 700 °C,

[0037] Fig. 8: a diagram showing the mass change of the Mo-ZnO sample during a heat treatment of a Mo-ZnO tablet at 800 °C for 200 hours,

[0038] Fig. 9: an SEM image of a microstructure (cross section) of a Mo-ZnO mixture after treatment at 800 °C for 200 hours,

[0039] Fig. 10: SEM images of a cross-section of a Mo-ZnO sample at different times, i.e. after one hour, 5 hours and 20 hours of heat treatment at 900 °C,

[0040] Fig. 11: a comparative juxtaposition of XRD diffractograms of differently heat-treated Mo-ZnO alloys with the same ZnO mass fraction and a heat-treated Mo-ZnO alloy with a lower ZnO mass fraction.

[0041] Fig. 12: SEM images of microstructures of molybdenum-zinc oxide alloys with different zinc oxide concentrations,

[0042] Fig. 13: Energy-dispersive X-ray spectroscopy (EDX) images for the analysis of the behavior of a molybdenum-zinc oxide alloy with a zinc oxide content of 46 wt% after heat treatment at 800 °C after 20 hours,

[0043] Fig. 14: a comparison of SEM images of microstructures (cross-section) of various Mo-ZnO samples with different weight percent ZnO content after 20 hours of heat treatment at 800 °C and SEM images of microstructures (cross-section) of the same samples after 20 hours of heat treatment at 900 °C, and

[0044] Fig. 15: SEM images of a cross-section of a Mo-ZnO sample with a zinc oxide content of 31 wt% at different times, i.e., after one hour, five hours, and twenty hours of heat treatment at 800 °C. As already mentioned, the Mo-ZnO system was investigated in more detail with regard to the formation of zinc molybdate, and the results of this investigation are presented below.

[0045] First, Mo and ZnO powders were mixed in a mass ratio of 1.18:1 and then compacted into a solid. The tablets produced in this way were subjected to various heat treatments in the temperature range between 500 °C and 900 °C, which is critical for molybdenum alloys, to test whether the zinc oxide combines with the volatile molybdenum trioxide under these conditions, thus resulting in molybdate formation. Finally, the heat-treated samples were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy to investigate the formation and influence of zinc molybdate in powder-metallurgically produced Mo-ZnO alloys and, furthermore, to determine whether catastrophic oxidation failure in molybdenum alloys can be mitigated or even completely prevented up to 900 °C.

[0046] Table 1 shows a mass comparison of the alloy before and after heat treatment, and after 1 hour and 5 hours of heat treatment.

[0047] Table 1

[0048] It was demonstrated that no catastrophic material failure occurs within the critical temperature range of 500 °C to 900 °C, as only a slight increase in mass was observed. This is due to the continuous absorption of oxygen and the associated reaction of zinc oxide with the molybdenum trioxide, which forms as a result of the oxidation of molybdenum, to form zinc molybdate. Figures 1A and 1B show diagrams illustrating the changes in the material composition during heat treatment: Figure 1A shows the composition after one hour, and Figure 1B shows the composition after five hours of heat treatment. The material composition refers to the corresponding phase fractions of molybdenum, zinc oxide, the two zinc molybates MoZnÜ₄ and Z^MosOs, and MoO₂, expressed in weight percent.

[0049] Figure 2 shows a comparative analysis of XRD diffractograms of an untreated Mo-ZnO tablet (see Figure a), a Mo-ZnO tablet treated at 800 °C (see Figure b), and a Mo-ZnO tablet treated at 900 °C (see Figure c). The XRD measurements confirmed that from approximately 700 °C, which corresponds to the approximate volatilization temperature of molybdenum trioxide, two different zinc-molybdates, ZnMoO₄ and Z⁻MosO₄, are formed. In the respective SEM cross-sections, a molybdate layer is visible at both 700 °C and 800 °C, which largely protects the underlying material from further oxidation. Figure 3A shows a scanning electron microscope (SEM) image of a cross-section of a Mo-ZnO sample after 5 hours of heat treatment at 800 °C, revealing the formation of a zinc-molybdate surface layer. Figure 3B shows the result of an energy-dispersive X-ray spectroscopy (EDX) analysis of a sample shown in Figure 3A.Figure 3A shows the rectangular area marked in Figure 4A. Areas originally colored green in the EDX analysis appear as light areas in the black-and-white image, indicating the molybdenum of the alloy, while areas originally colored red, which appear dark in the black-and-white image and are therefore hardly distinguishable from the zinc-molybdate of the surface layer, are attributable to zinc and oxygen. Advanced zinc-molybdate surface layer formation is also visible in Figure 4A, which shows a SEM image of a cross-section of a Mo-ZnO sample after 20 hours of heat treatment at 800 °C. Figure 4B shows the corresponding result of an analysis of a rectangular area of ​​this cross-section marked in Figure 4A using energy-dispersive X-ray spectroscopy (EDX).

[0050] Due to the presence of light and dark regions, the solids were examined separately using Raman spectroscopy. Figure 5A shows a Raman spectrum, and Figure 5B shows a corresponding light microscopic image with a crosshair indicating the light region under investigation. Figure 6A shows a Raman spectrum, and Figure 6B shows a corresponding light microscopic image with a crosshair indicating the dark region under investigation. Surprisingly, the light regions in Figure 5B could be assigned to zinc oxide, while the dark regions in Figure 6B are very likely Z⁻Mo⁻O₄. At a lower temperature, namely 700 °C, the light region corresponds to ZnMo⁻O₄, and the dark region indicates that the conversion to ZnMo⁻O₄ is not yet complete.

[0051] Figure 7A shows the result of a Raman spectroscopy investigation of a light surface area on the Mo-ZnO tablet after 5 hours of heat treatment at 700 °C, while Figure 7B shows, for comparison, a Raman spectrum of a dark surface area after a corresponding heat treatment. The numbers 1, 50, and 100 represent the number of measurements, each lasting 3 seconds.

[0052] Furthermore, cyclic oxidation tests were performed, whereby the Mo-ZnO tablet was subjected to heat treatment at 800 °C for 200 hours. Figure 8 shows the mass change of the Mo-ZnO sample during this heat treatment. The mass increase indicates that after an initial increase, a plateau is reached after approximately 50 hours. After 200 hours of heat treatment at a temperature of 800 °C, no further significant mass change occurs.

[0053] The corresponding SEM images, such as the one in Fig. 9, show that the initially formed zinc-molybdate layer has grown to approximately 50 pm. Furthermore, it can be seen that the transition zone between the base material and the zinc-molybdate layer is free of pores, indicating that a dense layer has formed in this area. Only in the upper region are some larger pores visible, which, however, apparently have no significant impact on the oxidation resistance.

[0054] Cyclic oxidation tests have shown that the addition of zinc oxide effectively protects molybdenum against oxidation in the critical temperature range, as a protective, dense zinc-molybdate layer forms. The results have demonstrated that Mo-ZnO alloys are oxidation-resistant for up to 200 hours at a temperature of 800 °C.

[0055] Furthermore, as already mentioned, experiments were also carried out at a temperature of 900 °C. Figure 10 shows three SEM images of a cross-section of a Mo-ZnO sample. Figure a) shows an SEM image after one hour, Figure b) an SEM image after five hours, and Figure c) an SEM image after 20 hours of heat treatment at 900 °C. The oxidation tests at a temperature of 900 °C show that the samples can withstand this temperature, as no melting of the zinc-molybdate coating was observed. Thus, the alloy developed here can be used without problems even at an operating temperature of 900 °C.

[0056] After heat treatment, the mass increased analogously to the previous temperatures, although the mass increase in this case was not as pronounced, as shown in Table 1. It is noteworthy that, in contrast to the respective masses at lower temperatures, the masses after 1 hour (Figure a) and after 5 hours (Figure b) differ only slightly from each other, indicating a dense, closed surface layer at a relatively early stage.

[0057] The corresponding SEM images in Fig. 10 show that a layer only a few pm thick has already formed after one hour, which, however, appears to be more pronounced compared to the layers formed after one hour at lower temperatures. The thickness of the layer does not increase significantly until the end of 20 hours (see Figure c), as the layer is already relatively dense after one hour of heat treatment (see Figure a), especially in the interface between the substrate and the layer. This suggests faster reaction kinetics due to the higher temperature; that is, the reaction between MoOs and ZnO proceeds more quickly, and consequently, a dense layer can form more rapidly on the surface, almost completely preventing further oxygen penetration at an earlier stage.

[0058] The phase fractions shown in Figures 1A and 1B at a temperature of 900 °C indicate that the top layer consists largely of Z^MosOs and the proportion of ZnMoÜ4 is very low. It is noteworthy that the proportion of molybdenum dioxide (MoO2) at 900 °C is lower after both one hour and five hours compared to the corresponding proportion at 800 °C. The proportions of molybdenum (Mo) and zinc oxide (ZnO) increase slightly, suggesting that the phase fractions of the two starting materials increase again above 800 °C. Consequently, the diffractograms in Figure 2 show no significant differences after five hours at temperatures of 800 °C and 900 °C.

[0059] In summary, the experiments regarding the formation and influence of zinc molybdate in Mo-ZnO alloys clearly show that catastrophic material failure in the temperature range critical for molybdenum alloys could be completely prevented.

[0060] After demonstrating that the addition of zinc oxide is sufficient to significantly improve the oxidation resistance of molybdenum in a range of 500 °C to 900 °C and to completely prevent catastrophic oxidation failure—based on a targeted chemical reaction between zinc oxide and the volatile molybdenum trioxide to form zinc molybdate—it was further discovered that zinc oxide, upon the addition of silicon, also reacts with silicon dioxide (SiO2), forming zinc silicate (Zn2SiO4). Zinc silicate offers numerous advantages, which will be briefly explained below. Zinc silicate has a melting point close to 1500 °C, which makes it suitable for

[0061] This makes zinc silicate extremely interesting for high-temperature applications, especially as a new material for gas and aircraft turbines. Furthermore, it is highly effective against atmospheric corrosion and possesses self-healing properties. Zinc silicate forms from silicon dioxide (SiO2) and zinc oxide (ZnO), typically at temperatures between 950 °C and 1000 °C. Due to the aforementioned self-healing properties and the high coefficient of thermal expansion of zinc, which prevents the layers from cracking on other metals, zinc silicate is a promising material for a coating in molybdenum alloys. Further experiments should determine the critical zinc oxide concentration in the composition of the starting materials for the formation of a protective zinc-molybdate coating in powder-metallurgically produced Mo-ZnO alloys. The figure...Figure 11 shows a comparative juxtaposition of XRD diffractograms of an untreated Mo-ZnO alloy in Figure a), a Mo-ZnO alloy heat-treated and oxidized for 20 hours at 800 °C in the presence of oxygen in Figure b), and a Mo-ZnO alloy heat-treated and oxidized for 20 hours at 900 °C in Figure c), all of which have a ZnO mass fraction of 31%, as well as a Mo-ZnO alloy heat-treated and oxidized for 5 hours at 800 °C in the presence of oxygen with a ZnO content of 29% in Figure d).

[0062] The results show that a critical concentration is reached at a composition of approximately 69 wt% molybdenum and 31 wt% zinc oxide, that is, in a composition where molybdenum is present in a 2.23-fold mass excess compared to zinc oxide. A significant reduction in the zinc oxide content below this threshold can impair the protective effect and reduce the alloy's ability to effectively resist oxidation. Very similar XRD diffractograms were obtained for compositions with a higher zinc oxide content, for example, a mass fraction of 46 wt% zinc oxide and 54 wt% molybdenum, corresponding to a 1.17-fold mass excess of molybdenum compared to zinc oxide, or 33 wt% zinc oxide and 67 wt% molybdenum, corresponding to a 2.03-fold mass excess of molybdenum compared to zinc oxide.The molybdenum alloys obtained from all three different compositions resisted oxidation and formed a protective zinc-molybdate coating. In contrast, the alloy with only 29 wt% zinc oxide and 71 wt% molybdenum, corresponding to a 2.45-fold mass excess of molybdenum relative to zinc oxide, resulted in catastrophic oxidation failure, as evidenced by the appearance of MoOs reflections in Figure d) of Fig. 11. Since all the diffractograms of the alloys that resisted oxidation and formed a protective zinc-molybdate coating are largely similar, the composition with a 31 wt% zinc oxide content serves as a representative example, to which Figures a), b), and c) of Fig. 11 are assigned. Figure a) of Fig. 11 shows the X-ray diffractogram of the untreated Mo-ZnO alloy. Figure b) of Fig.Figure 11 shows a Mo-ZnO alloy that was heat-treated and oxidized for 20 hours at 800 °C in the presence of oxygen. The diffractogram in Figure b) shows reflections of the starting materials molybdenum (Mo) and zinc oxide (ZnO) at 800 °C, as well as reflections of the two zinc molybdates, ZnMoÜ4 and Z^MosOs. As Figure c) in Figure 11 shows, the reflections of ZnMoÜ4 disappear completely at 900 °C, suggesting that the Z^MosOs phase is the predominant molybdate phase at higher temperatures. The proportions of the different phases in the investigated Mo-ZnO alloys are listed in Table 2 and show the influence of temperature and ZnO concentration. The following abbreviations are assigned to the various Mo-ZnO alloys according to their weight percent content of zinc oxide: Mo-46ZnO, Mo-33ZnO, Mo-31ZnO and Mo-29ZnO.More precisely, Table 2 shows the phase fractions of Mo-46ZnO, Mo-33ZnO, Mo-31ZnO, and Mo-29ZnO as a function of the ZnO concentration. Table 2 illustrates that a decrease in ZnO concentration leads to an increase in the fraction of ZnMoÜ4, which is the dominant phase at lower ZnO concentrations. Increasing the temperature to 900 °C leads to the complete conversion of ZnMoÜ4 to Zn2Mo3Os, so that the latter is the final reaction product of this system. Table 2:

[0063] Table 3 shows the phase fractions of ZnMoCM, Z^MosOs, MoO2, and MoOs in the alloy designated Mo-31ZnO as a function of oxidation time. In particular, Table 3 illustrates the development of the two molybdate phases in the alloy with increasing oxidation time. At a temperature of 800 °C, the proportion of Z^MosOs increases continuously and reaches a plateau over time. However, the ZnMoO4 phase is still present in the alloy even after 20 hours, indicating that the zinc-molybdate layer consists of both molybdate phases at this temperature. At higher temperatures, the amount of the ZnMoO4 phase is significantly lower at the beginning of the heat treatment than at 800 °C and disappears completely after 20 hours. On the other hand, the Z^MosOs content continues to increase, so that after 20 hours the top layer consists entirely of this molybdate phase, regardless of the zinc oxide concentration.

[0064] Table 3:

[0065] Figure 12 shows four electron microscope (SEM) micrographs of the microstructures of the various molybdenum-zinc oxide alloys a) Mo-46ZnO, b) Mo-33ZnO, c) Mo-31ZnO, and d) Mo-29ZnO, acquired using electron backscatter diffraction (BSE). These images illustrate how the microstructure changes depending on the zinc oxide concentration. The heat treatment temperature was 800 °C. The holding time for the alloys Mo-46ZnO, Mo-33ZnO, and Mo-31ZnO (see Figures a), b), and c) was 20 hours, while the holding time for the alloy Mo-29ZnO (see Figure d) of Figure 12 was only 5 hours. In the alloys Mo-46ZnO, Mo-33ZnO, and Mo-31ZnO, as shown in images a), b), and c), a homogeneous zinc-molybdate coating has formed, consisting of the two zinc-molybdates ZnMCk and Z^MosOs. Fig.Figure 13 shows an EDX analysis of the alloy Mo-46ZnO at 800 °C after 20 hours, which confirms that the formed surface layer consists of a superposition of molybdenum, zinc, and oxygen particles, which can be explained by the formation of zinc-molybdate. A reduction of the ZnO concentration to 29 wt%, as is the case for the alloy Mo-29ZnO shown in Figure d) of Figure 12, already leads to a cracked and porous zinc-molybdate surface layer. This damage is due to the density difference between zinc oxide (ZnO), which has a density of 5.6 g / cm³. 3 exhibits, and zinc molybdate (ZnMoCM) with a density of 4.3 g / cm³ 3 This is due to the fact that, according to Table 2, a lower ZnO content primarily leads to the formation of ZnMoO4, while a higher ZnO content results in the formation of more Zn2MosO8. Since the density of ZnMoO8 is 6.3 g / cm³ 3Since the oxidation state is closer to that of ZnO, the formation of Z^MosOs in alloys with a higher ZnO content does not lead to significant volume changes. In contrast, the predominant formation of ZnMoO4 during oxidation in the Mo-29ZnO alloy leads to volume expansion, generating excessive internal stresses within the layer, which subsequently leads to cracking. This causes the layer to rupture, allowing oxygen to penetrate deeper into the material. This, in turn, leads to renewed evaporation of MoOs and, consequently, to further zinc-molybdate formation within the material, thus initiating catastrophic material failure. In the Fig.Figure 14 shows SEM images of microstructures (cross-sections) of various Mo-ZnO samples with different weight percent ZnO content, namely Mo-46ZnO in Figure a), Mo-33ZnO in Figure c), and Mo-31ZnO in Figure e), after 20 hours of heat treatment at 800 °C. These images are compared to SEM images of microstructures (cross-sections) of the same samples, i.e., Mo-46ZnO in Figure b), Mo-33ZnO in Figure d), and Mo-31ZnO in Figure e), after 20 hours of heat treatment at 900 °C. The cross-sections at 800 °C, see Figures a), b), and e) of Figure 14, show the formation of two different zinc-molybdate layers, recognizable by the contrasting gray levels in the alloy interfaces, which are indicated by arrows. Based on the data in Table 2, it can be concluded that these individual layers consist of ZnMoÜ4 and Z^MosOs. At 900 °C, see figures b), d) and f) of the Fig.14, the presence of these two layers is less pronounced, since the layer according to Table 2 consists entirely of Z^MosOs at this temperature.

[0066] The development of the zinc-molybdate layer growth with increasing oxidation time for the Mo-31ZnO alloy is shown in Fig. 15, which depicts SEM images of a cross-section of this Mo-ZnO sample, containing 31 wt% zinc oxide, at various times, i.e., after one hour, five hours, and twenty hours of heat treatment at 800 °C. Initially, after about one hour of heat treatment, the layer thickness is in the range of approximately 13 to 18 pm, as shown in Figure a). The distribution of the particles appears more uniform at the beginning, indicating limited diffusion of the elements and subsequent molybdate formation within this short time. After five hours, the layer thickness increases and reaches values ​​between 18 and 27 pm, as shown in Figure b) of Fig. 15.At this stage, the microstructure exhibits a higher density of larger particles, indicating increased particle growth with characteristic sinter necks, which in turn suggests advanced sintering of the alloy. A further increase in the oxidation time to 20 h leads to further layer growth, so that the thickness at this stage is approximately 22 to 33 pm (see Figure c) of Fig. 15. This suggests that longer oxidation promotes the growth of the protective zinc-molybdate layer. On the other hand, the continued layer growth indicates that the formation of the outer capping layer is not yet complete, as the molybdate layer is still not dense even after 20 hours of heat treatment.Consequently, the coating still contains pores, micropores or even nanopores through which oxygen can continue to penetrate the material, enabling further oxidation of molybdenum and the formation of zinc-molybdate.

[0067] List of abbreviations used

[0068] EDX Energy-dispersive X-ray spectroscopy

[0069] SEM scanning electron microscopy wE arbitrary unit (English au = arbitrary unit)

[0070] XRD X-ray diffraction

Claims

Patent claims 1. Molybdenum alloy for use in the temperature range of 500 °C to at least 900 °C, obtainable by a powder metallurgy process using molybdenum as the starting element or a molybdenum-based base alloy and at least one metal oxide as starting materials, and heat treatment at a temperature in the range of 500 °C to a maximum of 1000 °C in an oxidizing atmosphere until a closed metal-molybdate layer is formed, wherein the metal of the metal-molybdate layer is derived from the at least one metal oxide selected from the group consisting of zinc oxide (ZnO), calcium oxide (CaO), a manganese oxide (MnÜ2, Mn2Ü3), magnesium oxide (MgO), and nickel oxide (NiO), and wherein molybdenum and the at least one metal oxide are mixed together in a mass ratio in which molybdenum has the same mass or a mass excess compared to the at least one metal oxide.

2. Molybdenum alloy according to claim 1, characterized in that the mass of the at least one metal oxide is at least 30% of the total mass of the starting materials of the powder metallurgy process.

3. Molybdenum alloy according to claim 2, characterized in that molybdenum is present in the starting materials in a mass excess of 1.16 to 1.18 times that of the metal oxide.

4. Molybdenum alloy according to one of claims 1 to 3, characterized in that the metal-molybdate layer of the molybdenum alloy is formed from the starting materials molybdenum and zinc oxide, wherein the formed metal-molybdate layer consists of one or more zinc-molybdate compounds.

5. Molybdenum alloy according to claim 4, characterized in that the zinc-molybdate layer consists of ZnMoCk and / or Z^MosOs.

6. Molybdenum alloy according to one of claims 1 to 3, characterized in that the metal-molybdate layer of the alloy is formed from molybdenum and a manganese oxide as starting materials, wherein manganese(III) oxide (Mn2Ü3) or manganese(IV) oxide (MnC ) is used.

7. Molybdenum alloy according to one of claims 1 to 6, characterized in that one or more metallic and / or semi-metallic alloying elements X are added to the molybdenum alloys as elements or in the form of oxides of the elements.

8. Molybdenum alloy according to claim 7, characterized in that the added alloying element(s) X are selected from the elements aluminium, nickel, titanium, zirconium, vanadium, niobium, tantalum, tungsten, chromium, hafnium, yttrium, lanthanum, boron and silicon.

9. Molybdenum alloy according to claim 8, characterized in that in the molybdenum alloy, in addition to the metal-molybdate layer, at least one or more further phases of a compound of one or more selected further alloying elements X with or without molybdenum are formed.

10. Molybdenum alloy according to claim 8 or 9, characterized in that the metal-molybdate layer of the molybdenum alloy is formed from the starting materials molybdenum and zinc oxide (ZnO), wherein silicon is added as an element or in the form of silicon dioxide (SiO2) as a starting material in addition to molybdenum and zinc oxide.

11. Molybdenum alloy according to claim 10, characterized in that the molybdenum alloy comprises a zinc silicate phase (Zn2SiÜ4 and / or ZnSiOs) formed from silicon and zinc oxide (ZnO).

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