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US-12623971-B2 - Aqueous suspension containing metal carbide particles

US12623971B2US 12623971 B2US12623971 B2US 12623971B2US-12623971-B2

Abstract

The present invention relates to aqueous suspensions containing 30 to 95 wt.-% metal carbide particles and a dispersant, and to a process for coating substrates using said aqueous suspensions. The invention also relates to the coated substrates that can be produced by the process according to the invention and to the uses thereof.

Inventors

  • Stanislaus Schwanke
  • Stephan Müller
  • Elke Meissner
  • Boris Epelbaum
  • Christian Reimann
  • Jochen Friedrich
  • Lucas Becker

Assignees

  • Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V.

Dates

Publication Date
20260512
Application Date
20190130
Priority Date
20180206

Claims (19)

  1. 1 . An aqueous suspension comprising at least one metal carbide particle selected from the group consisting of carbides of hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, and at least one dispersant, wherein the proportion of the at least one metal carbide particle present is present in the range from 60% to 95% by weight based on the total weight of the suspension, and wherein solvents other than water are present in the aqueous suspension in an amount of up to 2% by weight or no solvents other than water are present in the aqueous suspension.
  2. 2 . The aqueous suspension of claim 1 , wherein: the at least one metal carbide particle has an average particle size in the range from 0.05 to 25 μm; the at least one metal carbide particle has a content of individual elemental impurity of <300 ppm; and/or the dispersant is selected from the group consisting of polyacrylic acid, tetrabutylammonium hydroxide, and mixtures thereof.
  3. 3 . The aqueous suspension of claim 1 , which comprises at least one additive selected from the group consisting of a base, a defoamer, a sintering aid, and mixtures thereof.
  4. 4 . The aqueous suspension of claim 3 , wherein the base is sodium hydroxide solution, the defoamer is a fatty alcohol polyalkylene glycol ether, and/or the sintering aid is cobalt, silicon, or a mixture thereof.
  5. 5 . The aqueous suspension of claim 1 , wherein: the proportion of metal carbide particles is in the range from 60% to 90% by weight based on the total weight of the suspension; and/or the proportion of the dispersant is in the range from 0.05% to 5% by weight based on the total weight of the suspension; and/or the proportion of the at least one additive is in the range from 0% to 10% by weight based on the total weight of the suspension.
  6. 6 . A process for coating a substrate comprising the following steps: i) providing a substrate; ii) providing an aqueous suspension comprising at least one metal carbide particle selected from the group consisting of carbides of hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, and at least one dispersant, wherein the proportion of the at least one metal carbide particle present is present in the range from 60% to 95% by weight based on the total weight of the suspension, and wherein solvents other than water are present in the aqueous suspension in an amount of up to 2% by weight or no solvents other than water are present in the aqueous suspension; iii) applying the suspension from step ii) by spraying on to the surface of the substrate from step i); and iv) drying the applied suspension, resulting in the formation of a coating on the surface of the substrate.
  7. 7 . The process of claim 6 , wherein the substrate is selected from the group consisting of graphite, materials having an adjusted coefficient of thermal expansion, and mixtures thereof.
  8. 8 . The process of claim 6 , further comprising: v) pretreating the substrate provided in step i) prior to step iii), with the pretreatment effected by a measure selected from the group consisting of mechanical roughening of the surface, thermal pretreatment of the surface, chemical treatment of the surface, and combinations thereof, and subsequent cleaning, in particular by means of sonication; and vi) sintering of the coating obtained after step iv).
  9. 9 . The process of claim 6 , wherein step iv) is carried out at a temperature in the range from 100 to 600° C.
  10. 10 . The process of claim 9 , comprising carrying out step iv) over a period of 5 to 40 hours.
  11. 11 . The process of claim 10 , wherein, in step iv), the temperature is increased in stages.
  12. 12 . The process of claim 11 , wherein, in step iv) the temperature is increases as follows: (1) 140 to 160° C. for 2.5 to 3.5 hours; then (2) 180 to 220° C. for 1.5 to 2.5 hours; then (3) held for 2 hours at the temperature from (2); then (4) 200 to 250° C. for 1.5 to 2.5 hours; then (5) 310 to 350° C. for 4.5 to 5.5 hours; then (6) 330 to 350° C. for 1.5 to 2.5 hours; then (7) held for 2 hours at the temperature from (6); then (8) 380 to 420° C. for 3.5 to 4.5 hours; then (9) 430 to 470° C. for 1.5 to 2.5 hours.
  13. 13 . The process of claim 8 , wherein step vi) is carried out at a temperature in the range from 2000 to 2600° C. over a period of 1 to 10 hours; step vi) is carried out at a pressure in the range from 500 to 900 torr; and/or step vi) is carried out under inert gas.
  14. 14 . The process of claim 8 , wherein the green density of the coating after step iv) and/or prior to step vi) is at least 50%; and/or the coating after step iv) or vi) has an impurity content of less than 300 ppm; and/or the coating after step iv) or vi) has an open porosity of less than 5%.
  15. 15 . The process of claim 6 , wherein the thickness of the coating after step iv) or vi) is in the range from 20 to 500 μm; and/or the pH of the aqueous suspension prior to step iii) is in the range from 5 to 10.
  16. 16 . A coated substrate produced by a process for coating a substrate comprising: i) providing a substrate; ii) providing an aqueous suspension comprising at least one metal carbide particle and at least one dispersant, wherein the proportion of the at least one metal carbide particle is in the range from 60% to 95% by weight based on the total weight of the suspension, wherein the at least one metal carbide is selected from the group consisting of carbides of hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten; and wherein solvents other than water are present in the aqueous suspension in an amount of up to 2% by weight or no solvents other than water are present in the aqueous suspension; iii) applying by spraying on the suspension from step ii) to a surface of the substrate from step i); and iv) drying the applied suspension, resulting in the formation of a coating on the surface of the substrate.
  17. 17 . The coated substrate of claim 16 , wherein the thickness of the coating is in the range from 20 to 500 μm.
  18. 18 . A process of growing crystals comprising utilizing the coated substrate of claim 17 as a carbidic material.
  19. 19 . The process of growing crystals according to claim 18 , comprising a physical vapor-phase process, an epitaxy process, or crucible formation.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This patent application is the U.S. national phase of International Application No. PCT/EP2019/052235, filed on Jan. 30, 2019, which claims the benefit of German Patent Application No. 10 2018 201 771.9, filed Feb. 6, 2018, the disclosures of which are incorporated herein by reference in their entireties for all purposes. The present invention relates to aqueous suspensions comprising metal carbide particles and dispersants and to a process for coating substrates using these aqueous suspensions. The invention further relates to the coated substrates producible by the process according to the invention and to the uses thereof. High-melting metal carbides, for example carbides of titanium, zirconium, hafnium, niobium, tantalum, chromium, molybdenum, tungsten, and silicon, are characterized by their high mechanical, chemical, and thermal stability. Carbidic materials are consequently widely used in the form of solid ceramic bodies or in the form of coatings. For example, carbidic materials are used in the production of materials at high temperatures and/or in chemically aggressive environments, in the production of cutting tools or else engine nozzles, and in crystal growing. However, their mechanical hardness makes metal carbides difficult to process. This means that solid shaped bodies having relatively complex geometries can be produced from these materials only with difficulty and at considerable cost. For this reason, structural components are mostly only coated with metal carbides. Commonly used coating processes are the CVD (chemical vapor deposition) and PVD (physical vapor deposition) processes. However, the coatings obtainable by these processes are generally only a maximum of a few micrometers thick, which, because of poor long-term stability, is not sufficient for certain applications, for example in the production of silicon carbide or aluminum nitride crystals. The prior art also includes descriptions of metal carbide coatings produced via a wet-ceramic process in which an organic suspension of metal carbide particles is applied to the components to be coated by painting, spraying or dipping, followed by a sintering process. US 2013/0061800 A1 describes a highly thermally stable element that contains a graphite substrate comprising isotropic graphite. The highly thermally stable element also includes a carbide coating that contains a carbide such as tantalum carbide. In addition, processes for producing said element are described in which the carbide particles are deposited on the substrate from a suspension. This is done using suspensions with an organic solvent as the liquid phase. Wet-ceramic processes for coating graphite with tantalum carbide are also described by D. Nakamura, T. Kimura, T. Narita, A. Suzumura, T. Kimoto, and K. Nakshima in the Journal of Crystal Growth, vol. 478, 2017 on pages 163 to 173, and by D. Nakamura, K. Shigetoh, and A. Suzumura in the Journal of the European Ceramic Society, vol. 37, 2017 on pages 1175 to 1185. In the published processes, tantalum carbide is deposited from suspensions based on organic solvents. The wet-ceramic processes known from the prior art allow the production of relatively thick layers in which the layer is in some cases several hundred micrometers thick. In contrast to layers produced by CVD or PVD processes, layers produced by wet-ceramic processes have an isotropic texture with a random particle orientation, which results in reduced susceptibility to cracking and enhancement of the diffusion pathway for substrate-damaging species. Suspensions based on organic solvents do, however, have important disadvantages. In addition to ecological and health considerations arising from the toxicity of organic solvents, the use of such suspensions also entails the safety problem of highly flammable spray mists. Moreover, the organic solvents must be removed by pyrolysis. This results in the unwanted introduction of foreign matter into the coating. What is more, controlled application of the suspension is not possible with the known suspensions, particularly in the case of spray processes, since the suspension properties can fluctuate due to evaporation of the solvent during said process, which means that over time it becomes impossible to obtain homogeneous layers. It would additionally be desirable to be able to influence the ratio of open to closed pores in a carbide coating, which is possible only to a limited degree with the known suspensions based on organic solvents. A further desirable objective is the ability to achieve a coating having a greater depth of infiltration from the substrate surface. Based on this, the object of the present invention was to provide suspensions that are not associated with any ecological, health, and safety problems. The production from the suspensions of coatings that are very pure and do not require a pyrolysis step should also be possible. Using the provided suspensions, it