HK1069423B - Method for coating objects - Google Patents

Description

The invention relates to a process for coating articles of valve metals of Al, Mg, Ti, Nb and/or Zr or their alloys and the articles obtained thereby.

EP 0 545 230 A1 concerns a process for the production of modified oxide-ceramic layers on barrier-forming metals and articles obtained therefrom. To increase the thickness and wear resistance of oxide-ceramic layers on barrier-forming metals, plasma anodic anodic oxidation is carried out in a chloride-free electrolyte bath with a pH of 2 to 8 at a constant current density of at least 1 A/dm2 until the voltage reaches a final value. On articles made of aluminium or aluminium alloys an oxide-ceramic layer consisting of corundum can be produced.

For many applications, high-strength valve metal components must be corrosion resistant and wear-resistant even under extreme conditions, which is achieved by providing such objects with an oxide-ceramic layer with a wide-mesh capillary system, introducing fluoropolymer particles at least one dimension smaller than the diameter of the capillaries and subjecting the object with the pre-filled capillary system to changing pressure conditions.

DE 41 24 730 C2 concerns a process for depositing fluoropolymers in microporous, anodically oxidized surfaces of aluminium or aluminium alloys characterised by depositing an aqueous suspension of fluoropolymers or their precursors with a particle size of 1 to 50 nm into the capillaries perpendicular to the metal of a hardeloxal layer.

DE 42 39 391 C2 covers articles of aluminium, magnesium or titanium with an oxide-ceramic layer filled with fluoropolymers and the method of their manufacture. It describes articles of the barrier-forming metal with a thin adhesive barrier layer on the metal, on which a sintered dense oxide-ceramic layer is placed and on which an oxide-ceramic layer with a wide mesh capillary system, essentially filled with fluoropolymers, is placed. The oxide-ceramic layer has a thickness of 40 to 150 μm. Examples of such articles are rotor floors for turbocharged molecular pumps, turbines for diesel or petrol engines, vacuum or vacuum tubes for cylinders, plastic balls for cylinders, and aluminium alloys.It describes the introduction of particles of the fluoropolymer or its precursor, if they are not liquids, into the outer oxide-ceramic layer as a solution or suspension in an appropriate solvent. The essential point of this description is to expose the particles of fluoropolymers in an appropriate solvent to changing pressure conditions, for which an impregnation system is suitable, whereby the air is first removed from the capillary system of the oxide-ceramic layer by means of a vacuum and then under vacuum, the particles enter the pores and after the vacuum is lifted, the atmospheric pressure is applied in pores and sometimes also fine adjustments are achieved.

In particular, the polymers and copolymers of tetrafluoroethylene, hexanfluoropropene, vinylidene fluoride, vinyl fluoride and trifluorochloroethylene are described as particularly suitable fluoropolymers, which are known to be practically insoluble in any solvent and are therefore expected to be introduced to the surface in the form of dispersions for polymers according to DE 42 39 391 C2.

A similar process is described in Japanese patent JP 2.913.537, a corrosion resistant structure characterised by the inclusion of a layer of NiP alloy of approximately 20 μm thickness in an aluminium/alloy part of a turbomolecular pump for the discharge of chlorine gas in semiconductor manufacturing plants, which comes into contact with chlorine gas, and the formation of a fluorine resin protective layer on the layer of fluorine resin by immersing a rotor and a stator of the turbomolecular pump in a liquid to form the escape layer of the resin and then drying them.

The common feature of the above state of the art is that the fluoropolymers are mainly present on the outer surface of the oxide-ceramic layer but penetrate only to a small extent into the branches.

The present invention is therefore intended to improve the uniformity, coating and thus sealing of articles, in particular oxide ceramic coatings.

The above problem is solved in a first embodiment by a process of coating objects made of valve metals of Al, Mg, Ti, Nb and/or Zr or their alloys with a thin metal barrier layer and an oxide-ceramic layer on top of which the surface is coated with fluoropolymers, characterised by introducing the fluoropolymers in solution form by vacuum impregnation into the capillary system of the oxide-ceramic layer and drying after removal of the non-moisturising parts of the solution.

The further treatment of oxide or ceramic layers, especially those produced by anodizing, by means of vacuum impregnation with solutions of fluoropolymers can significantly improve the leak-tightness properties of the protective layers compared with the state of the art. Another advantage in the application of the polymers described is their extremely high resistance to aggressive and corrosive media. These media can be gaseous, for example when used in turbomolecular pumps in plasmas, but also include liquids or vapors of acids or alkalis.

In the same way, it is also possible to impregnate fluoropolymers in solution without the need for any oxidative or ceramic coating. The surfaces treated in this way also have special characteristics such as the rejection of dirt or dust particles and the impenetrability of media such as water, oils or other liquids.

The present invention makes it possible to improve the uniformity of the wetting significantly compared with the state of the art by means of vacuum impregnation of the above-mentioned layers, whereby the dissolved fluoropolymer penetrates the pores or microscopic cavities of the layers.

The advantages of the coatings according to the present invention lie in particular in their very low surface energy, which results in an optimal barrier effect against almost all solvents, including in particular solvents, oils (including silicone oils) and water-based liquids. Solids are also very difficult to deposit on the surface of the film. The same property also results in a very good adhesion to the valve metals.

For the purposes of the present invention, aluminium, magnesium, titanium, niobium or zirconium and alloys thereof are used as valve metals.

The main features of this approach are aluminium and aluminium alloys, which are frequently used for the manufacture of rotors in turbomolecular pumps.

For the purposes of this invention, aluminium and alloys thereof are pure aluminium and alloys AlMn; AlMnCu; AlMg1; AlMg1,5; E-AlMgSi; AlMgSi0,5; AlZnMgCu0,5; AlZnMgCu1,5; G-AlSi-12; G-AlSi5MG, G-AlSi8Cu3, G-AlCu4Ti; G-AlCu4TiMg.

For the purposes of the invention, in particular, the magnesium alloys of ASTM designations AS41, AM60, AZ61, AZ63, AZ81, AZ91, AZ92, HK31, QE22, ZE41, ZH62, ZK51, ZK61, EZ33, HZ32 and the knet alloys AZ31, AZ61, AZ80, M1 ZK60, ZK40 are suitable.

The use of pure titanium or titanium alloys such as TiAl6V4, TiAl5Fe2,5 and others is also possible.

The oxide-ceramic layer is made of a more or less gradient material, where the oxide-ceramic layer is densely sintered on the side of the barrier layer and has a wide-meshed capillary system on the opposite side.

Also used in the present invention are oxide-ceramic coatings with a thickness of 40 to 150 μm, in particular 50 to 120 μm, as also known from DE 42 39 391 C2.

The fluoropolymers that can be used for the purpose of the present invention are preferably selected from fluorinated epoxy polymers, sileythers, particularly fluoro-aliphatic sileythers, polyacrylates and/or urethane.

FluoradTM FC-405/60 is described as a concentrated solution of a fluoroaliphic silylether that is soluble in alcohol, ketones, acetate and also dissolves in water. FluoradTMFC-722 is described as a fluorinated acrylic polymer in a fluorinated solvent. As a thermoplastic, the agent does not dry out, making higher temperatures and exothermic reactions by UV or low temperature systems superfluous. FluoradTMFC-725 is described as a fluorinated acrylic polymer in butylacetate.

In particular, the fluoropolymers are preferred in the present invention to be applied in a layer thickness of 1 to 20 μm, in particular 1 to 5 μm. In the case of polyacrylates, a particularly low layer thickness is of particular advantage.

The method chosen depends on the fluoropolymers used and the requirements of the articles.

The dry vacuum/pressure impregnation process is the slowest and most complicated vacuum impregnation process. The vacuum impregnation process described below requires two tanks, one for the solution of the fluoropolymer and one for impregnating the articles/pieces: ■ vacuum in the impregnating container to remove air from the pores of the articles; ■ flood the solution of the fluoropolymer from the reservoir into the impregnating container until the workpieces still in the vacuum are flooded; ■ aerate the impregnating container and then apply pressure with compressed air; ■ use the overpressure to return the solution of the fluoropolymer to the reservoir and then vent it to normal pressure; ■ remove and rinse the articles; ■ perform further steps.

The dry/vacuum method is preferred for highly viscous solutions of fluoropolymers, and is particularly preferred for applications where the pores are very small and the requirements on the coated articles are unusually high.

The wet vacuum/pressure process requires only one tank. The objects are immersed in the solution of the fluoropolymer, which remains permanently in the impregnating vessel. The objects and the solution of the fluoropolymer are pressurised together under vacuum and then with compressed air: ■ vacuum in the impregnating vessel to remove air from the pores of the parts; ■ aerate at normal pressure and then pressurise the tank with compressed air; ■ apply normal pressure; ■ remove and rinse workpieces; ■ perform further process steps.

The wet vacuum/pressure method is recommended for example for impregnating objects with very small pores and for high density sinter metal parts.

The wet vacuum process is the simplest and fastest vacuum impregnating method. It is similar to the wet vacuum/pressure process, but the tank is not pressurized. Instead, the impregnating tank is simply aerated after the vacuum. The sealing material penetrates the parts at atmospheric pressure. The solution of the fluoropolymer flows into the vacuum created in the pores of the parts and flushes it out: ■ vacuum to remove air from the pores; ■ ventilate at normal pressure; ■ leave parts in the impregnating container briefly to allow the solution of the fluoropolymer to penetrate; ■ remove and rinse workpieces; ■ perform further steps.

Wet vacuum impregnation is by far the preferred method, and its simplicity and speed, as well as the advantage of low capital costs, make it the preferred method when the impregnation system is re-installed.

After vacuum impregnation, the non-wetting parts of the solution are removed and, if necessary, the solvent is removed at elevated temperatures.

To increase the thickness of the layer, it is of course also possible to repeat the steps of introducing and drying the fluoropolymers several times.

The invention also includes valve metal products obtained by the above process, particularly preferred for the purpose of the present invention, which are rotors for turbomolecular pumps, which are usually made of aluminium or aluminium alloys.

The present invention produces articles characterized by an extremely low surface optical conductivity, as shown by comparative measurements of the optical conductivity of untreated oxide layers and vacuum impregnated oxide layers.

The vacuum impregnation ensures that the pores in the oxide layer and thus the entire surface are completely filled.

This approach is particularly advantageous for pore measurements of plasma-chemically generated layers, especially hard-anodic oxide layers.

The traditional immersion treatment only reaches the wettable surface but does not penetrate the pores (in this case especially the pores of hardanoid layers).

For this purpose, tests are performed on plasma oxide layers which showed a difference:

A luminous conductivity of 42 μS was obtained with a submersion treatment with thermal aftertreatment at 110 +/- 10 °C for 30 min, compared with 7 μS for a vacuum impregnation according to the invention.

Examples of implementations: Example 1/Comparative example 1:

A sample sheet of aluminium alloy AlMgSi1 was anodized in a standard standard electrolyte and a layer thickness of 25 μm on average was determined.

The test sheet was then vacuum-impregnated in a solution of commercially available fluorinated acrylate FluradTM FC-732 at a pressure of < 0,1 mbar and then treated at +/- 10°C for 30 min, and the apparent conductivity was also determined.

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The following is the list of the categories of the product concerned:

A sample sheet of aluminium alloy as described in Example 1/Comparative Example 1 was coated with a 20 μm thick plasma-oxide layer, the sample was also dried and the apparent conductivity was determined at an average of 35 μS (Comparative Example 2).

The sample was then vacuum-impregnated and heat treated as described in Example 1 and the apparent conductivity was determined to be < 3 μS (Example 2).

Claims (8)

1. A process for the coating of objects made of valve metals selected from aluminum, magnesium, titanium, niobium and/or zirconium and their alloys with a thin barrier layer consisting of the metal and an oxide ceramic layer provided thereon whose surface has been coated with fluoropolymers, characterized in that the fluoropolymers are introduced into the capillary system of the oxide ceramic layer in the form of a solution by vacuum impregnation, followed by removing the non-wetting portions of the solution and drying.
2. The process according to claim 1, characterized by employing an oxide ceramic layer which consists of a densely sintered oxide ceramic layer on the barrier layer and, on top of said densely sintered oxide ceramic layer, an oxide ceramic layer having a wide-meshed interlinked capillary system which has been applied by plasma-chemical anodic oxidation.
3. The process according to claim 1 or 2, characterized by employing an oxide ceramic layer having a thickness of from 40 to 150 µm, especially from 50 to 120 µm.
4. The process according to any of claims 1 to 3, characterized by employing fluoropolymers which are selected from fluorinated epoxide polymers, silyl ethers, especially fluoroaliphatic silyl ethers, polyacrylates and/or urethanes.
5. The process according to any of claims 1 to 4, characterized in that the fluoropolymers are applied in a layer thickness of from 1 to 20 µm, especially from 1 to 5 µm.
6. The process according to any of claims 1 to 5, characterized in that the steps of introducing and drying the fluoropolymers are repeated several times.
7. An object consisting of valve metals and obtainable by a process according to any of claims 1 to 6.
8. The object according to claim 7, characterized by being a rotor of aluminum or aluminum alloys for turbo-molecular pumps.