DE102014211385B3 - Process for coating a substrate - Google Patents

Abstract

The invention relates to a method for coating a substrate comprising a light metal and / or molybdenum and / or tantalum, the layer being produced by means of plasma-chemical anodic oxidation in an aqueous electrolyte, wherein the aqueous electrolyte comprises: ammonium heptamolybdate or ammonium heptamolybdate hexyahydrate, iron citrate or ammonium iron (III) citrate, - a complexing agent.

Description

The invention relates to a method for coating a substrate for producing a highly absorbent layer.

Surface properties of components, in particular of light metal alloys can be influenced in a targeted manner by a variety of technical processes. Particularly electrochemical processes, such as anodic oxidation and its subgroup, the plasma-chemical oxidation, play an increasingly important role in achieving functional surface properties such as color, wear and corrosion protection.

In addition to the pure generation of functional layers, the methods used also increasingly focus on health and environmental aspects and legal requirements. So z. B. Cr (VI) -containing compounds will be allowed in the future only for exceptional applications.

For plasma chemical oxidation, plasma electrolytic oxidation, micro arc oxidation as well as anodic oxidation under spark discharge, a variety of methods are known which realize various functionalities on the light metals aluminum, magnesium, titanium and their alloys. Thus, these layers are used as implant coating, for wear and corrosion protection and for use in the optical industry as a black coating.

In the DE 10 2011 055 644 B4 describes a method for the production of black layers on aluminum alloys containing high silicon.

The DE 41 169 10 A1 describes a pretreatment of silicon-containing cast aluminum alloys using a mixture of hydrofluoric acid and nitric acid and a subsequent coating by means of plasma chemical anodic oxidation. In this case, a Cr-containing compound is used in the electrolyte composition.

From the DD 156 003 B1 For example, a method of surface treatment of titanium and titanium alloys by spark discharge anodic oxidation is known, wherein the aqueous electrolyte solution may include molybdate and also citrate.

From the DE 101 27 770 A1 is a method for producing corrosion protection layers on surfaces of magnesium or magnesium alloys known, in which the components to be provided with the protective layer as an electrode, preferably as an anode, are switched and oxidized in an electrolyte bath. In addition to phosphate compounds and aluminum compounds, the electrolyte bath also contains at least one compound of vanadium and / or molybdenum and / or manganese.

The invention is based on the object to provide an improved method for coating a substrate with a highly absorbent layer.

The object is achieved by a method having the features of claim 1.

Advantageous embodiments of the invention are the subject of the dependent claims.

• – Ammoniumheptamolybdat oder Ammoniumheptamolybdathexyahydrat,
• – Eisencitrat oder Ammoniumeisen(III)citrat,
• – einen Komplexbildner.

In a method according to the invention for coating a substrate comprising a light metal and / or molybdenum and / or tantalum, the layer is produced by means of plasma-chemical anodic oxidation in an aqueous electrolyte, wherein the aqueous electrolyte comprises:

• Ammonium heptamolybdate or ammonium heptamolybdate hexyahydrate,
• Iron citrate or ammonium iron (III) citrate,
• - a complexing agent.

By light metals are meant those metals and alloys whose density is less than 5 g / cm ³ . In particular, these are aluminum, magnesium and titanium, but also other alkali metals, alkaline earth metals and metals of the scandium group. However, the method is also suitable for coating substrates of molybdenum or tantalum.

As the complexing agent, at least one of ethylenediamine, citric acid, ethylenediaminetetraacetate (EDTA), diethylenetriaminepentaacetic acid (DTPA) and triethanolamine can be used.

In one embodiment of the invention, the aqueous electrolyte may further comprise sodium tungstate.

Furthermore, the aqueous electrolyte may comprise at least one phosphate, carbonate, borate or silicate to aid in film formation. In particular, as phosphate potassium dihydrogen phosphate and as carbonate sodium carbonate in question.

In one embodiment of the invention, the aqueous electrolyte further comprises ammonia water for adjusting certain pH values of the electrolytes.

• – ein Anteil von 25 g/l bis 30 g/l Kaliumdihydrogenphosphat,
• – ein Anteil von 15 g/l bis 20 g/l Natriumcarbonat,
• – ein Anteil von 17,5 ml/l bis 37,5 ml/l Ammoniakwasser (beispielsweise in Form eines Anteils von 70 ml/l bis 150 ml/l fünfundzwanzigprozentigem Ammoniakwasser),
• – ein Anteil von 5 g/l bis 15 g/l Ammoniumheptamolybdathexyahydrat,
• – ein Anteil von 2 g/l bis 10 g/l Natriumwolframat,
• – ein Anteil von 25 m/l bis 50 ml/l Ethylendiamin, und
• – ein Anteil von 10 g/l bis 25 g/l Ammoniumeisen(III)citrat verwendet.

In an exemplary embodiment of the invention, in the aqueous electrolyte:

• A proportion of 25 g / l to 30 g / l of potassium dihydrogen phosphate,
• A proportion of 15 g / l to 20 g / l of sodium carbonate,
• A proportion of 17.5 ml / l to 37.5 ml / l of ammonia water (for example in the form of a proportion of 70 ml / l to 150 ml / l of twenty-five per cent ammonia water),
• A proportion of 5 g / l to 15 g / l ammonium heptamolybdate hexyahydrate,
• A proportion of 2 g / l to 10 g / l of sodium tungstate,
• A proportion of 25 m / l to 50 ml / l of ethylenediamine, and
• A proportion of 10 g / l to 25 g / l of ammonium iron (III) citrate is used.

• – Gleichstrom oder Wechselstrom mit einer Stromdichte von 0,05 A/dm² bis 15 A/dm²,
• – Spannungen von 200 V bis 500 V (vorzugsweise gemessen am Ausgang einer Stromversorgung, so dass Leitungsverluste außer Betracht bleiben)
• – Elektrolyttemperatur von 15 °C bis 45 °C
• – pH-Wert des Elektrolyten von 9,0 bis 11,0, insbesondere 10,2
• – Leitfähigkeit des Elektrolyten von 20 mS/cm bis 40 mS/cm, insbesondere 33,7 mS/cm.

In one embodiment of the invention, the plasma-chemical anodic oxidation is carried out with the following parameters:

• - direct or alternating current with a current density of 0,05 A / dm ² to 15 A / dm ² ,
• - Voltages from 200 V to 500 V (preferably measured at the output of a power supply, so that line losses are ignored)
• - electrolyte temperature from 15 ° C to 45 ° C
• - pH of the electrolyte from 9.0 to 11.0, in particular 10.2
• - Conductivity of the electrolyte from 20 mS / cm to 40 mS / cm, in particular 33.7 mS / cm.

The pH and conductivity of the electrolyte can be influenced by the composition of the electrolyte.

The DC or AC can be pulsed at a frequency of 10 Hz to 2000 Hz. By pulsed energizing a more uniform deposition is achieved. The substrate to be coated can be immersed, for example, as an anode in the aqueous electrolyte. As a counter electrode, for example, a sheet of a high-alloy chromium-nickel steel or an aluminum alloy can be used. Subsequently, the substrate to be coated is black to a final voltage of 200 V to 500 V and a pulsed direct current, for example, with a current density of 0.5 A / dm ² to 15 A / dm ² and a frequency of 10 kHz to 1500 kHz coated. Alternatively, the use of unpulsed DC or pulsed or unpulsed AC is possible. When AC is used, two substrates to be coated may be immersed as electrodes in the electrolyte, and an AC voltage may be applied thereto.

In one embodiment of the invention, after the plasma-chemical anodic oxidation, a nanoscale (for example 50 nm to 200 nm thick) transparent layer, for example SiO _x by chemical vapor deposition (CVD), for example by feeding a silicon-containing precursor, in particular HMDSO in a flame or a plasma , are deposited on the substrate. The layers deposited by means of the plasma-chemical anodic oxidation (PCO) typically have roughnesses in the μm range. The chemical vapor deposition superimposes on the roughness of the PCO layer a roughness in the nanometer range, so that the absorption is further increased. The chemical vapor deposition can be carried out at atmospheric pressure or at low pressure.

In one embodiment of the invention, before the plasma-chemical anodic oxidation, a mechanical and / or chemical pretreatment of the substrate, for example by pickling, for roughening the surface of the substrate, for example in a range of 10 .mu.m to 100 .mu.m. In conjunction with the PCO layer and optionally the layer deposited by means of CVD, broadband scattering and absorption can be achieved by means of roughness in a plurality, that is to say at least two or three scaling stages, so that the degree of absorption is further increased. By means of the optically highly absorbent coating, a total reflection of the substrates coated therewith can be minimized.

Optionally, before the PCO coating, a pretreatment of the substrate in the form of a cleaning process and / or a chemical pickling process can be carried out.

The coating according to the invention is particularly suitable for components which are formed predominantly of light metal or light metal alloys, but also of molybdenum or tantalum.

By means of the method according to the invention, by means of plasma chemical oxidation for the material group of light metal alloys, a uniform functional black coating can be achieved even on complicatedly shaped components which have a low reflectance of, for example, more than 90% both in the visible wavelength spectrum and in the near infrared range.

The inventive method avoids impairment of the mechanical properties of the substrate in near-surface areas. Further omitted by the inventive method, the use of heavy metal-containing electrolyte components and chromium-containing compounds.

Embodiments of the invention will be explained in more detail below.

example 1

A substrate of a spray-compacted high-silicon-containing aluminum alloy with a silicon content of about 35% is degreased in an aqueous mildly alkaline cleaning solution. This is followed by a two-fold rinse with deionized water and a post-treatment in an ultrasonic bath with a frequency of 45 kHz over three minutes. After another rinsing step with deionized water, the substrate becomes an anode in an aqueous electrolyte consisting of KH ₂ PO ₄ 25 g / l Na ₂ CO ₃ 17.5 g / l (NH ₄ ) ₆ Mo ₇ O ₂₄ x 4H ₂ O 10 g / l Na ₂ WO ₄ 4 g / l C ₂ H ₈ N ₂ (99%) 50 ml / l NH ₄ OH (25%) 75 ml / l C ₆ H ₈ O ₇ · n Fe · nH ₃ N 15 g / l coated. The electrical parameters used were set as follows: Current density: 2 A / dm ² Frequency: 500 Hz end voltage: 250V

After dropping the current to 50% of the output current, the process can be stopped by turning off the external power source. The black oxide ceramic coating thus obtained has a thickness of 12 μm. The diffuse reflection of this layer is 4% at the wavelength of 540 nm.

Example 2

A substrate made of aluminum alloy AlMgSi1 is degreased in an aqueous mild alkaline cleaning solution. This is followed by rinsing twice with deionized water and after-treatment in an ultrasonic bath at a frequency of 45 kHz over three minutes. After another rinsing step with deionized water, the substrate becomes an anode in an aqueous electrolyte consisting of KH ₂ PO ₄ 25 g / l Na ₂ CO ₃ 17.5 g / l (NH ₄ ) ₆ Mo ₇ O ₂₄ x 4H ₂ O 10 g / l Na ₂ WO ₄ 10 g / l C ₂ H ₈ N ₂ (99%) 25 ml / l NH ₄ OH (25%) 75 ml / l C ₆ H ₈ O ₇ · n Fe · nH ₃ N 15 g / l coated. The electrical parameters used were set as follows: Current density: 5 A / dm ² Frequency: 1600 Hz end voltage: 280 V.

After dropping the current to 50% of the output current, the process can be stopped by turning off the external power source. The black oxide ceramic coating thus obtained has a thickness of 22 μm. The reflection of this layer is 6.7% at the wavelength of 1200 nm.

Example 3

A titanium alloy TiAl6V4 substrate is degreased in an aqueous cleaning solution. This is followed by rinsing twice with deionized water and after-treatment in an ultrasonic bath at a frequency of 45 kHz over three minutes. After another rinsing step with deionized water, the substrate becomes an anode in an aqueous electrolyte consisting of KH ₂ PO ₄ 25 g / l Na ₂ CO ₃ 17.5 g / l (NH ₄ ) ₆ Mo ₇ O ₂₄ x 4H ₂ O 10 g / l C ₂ H ₈ N ₂ (99%) 25 ml / l NH ₄ OH (25%) 75 ml / l C ₆ H ₈ O ₇ · n Fe · nH ₃ N 15 g / l coated. The electrical parameters used were set as follows: Current density: 5 A / dm ² Frequency: 1000 Hz end voltage: 280 V.

After dropping the current to 50% of the output current, the process can be stopped by turning off the external power source.

The black oxide ceramic coating thus obtained has a thickness of 40 μm. The diffuse reflection of this layer is 4.6% at the wavelength of 1200 nm.

After the plasma-chemical anodic oxidation, in all embodiments a nanoscale, transparent layer can be deposited on the substrate by means of chemical vapor deposition in order to further increase the absorption.

Before the plasma-chemical anodic oxidation, a mechanical and / or chemical pretreatment of the substrate for roughening its surface in a roughness range of 10 .mu.m to 100 .mu.m can be performed in all embodiments.

Claims (11)

A method of coating a substrate comprising a light metal and / or molybdenum and / or tantalum, wherein the layer is produced by plasma chemical anodic oxidation in an aqueous electrolyte, wherein the aqueous electrolyte comprises: ammonium heptamolybdate or ammonium heptamolybdate hexyahydrate, iron citrate or ammonium ferric citrate . - a complexing agent. Process according to claim 1, wherein at least one of the compounds ethylenediamine, citric acid, ethylenediaminetetraacetate, diethylenetriaminepentaacetic acid and triethanolamine is used as complexing agent. Method according to one of claims 1 or 2, wherein the aqueous electrolyte further comprises at least one phosphate, carbonate, borate or silicate. A method according to claim 3, wherein potassium dihydrogen phosphate is used as the phosphate or wherein sodium carbonate is used as the carbonate. The method of any one of the preceding claims, wherein the aqueous electrolyte further comprises ammonia water. The method of any one of the preceding claims, wherein the aqueous electrolyte further contains sodium tungstate. Method according to one of the preceding claims, wherein in the aqueous electrolyte: A proportion of 25 g / l to 30 g / l of potassium dihydrogen phosphate, A proportion of 15 g / l to 20 g / l of sodium carbonate, A proportion of 17.5 ml / l to 37.5 ml / l of ammonia water, A proportion of 5 g / l to 15 g / l ammonium heptamolybdate hexyahydrate, A proportion of 2 g / l to 10 g / l of sodium tungstate, A proportion of 25 m / l to 50 ml / l of ethylenediamine, and - A proportion of 10 g / l to 25 g / l ammonium iron (III) citrate is used. Method according to one of the preceding claims, wherein the plasma-chemical anodic oxidation is carried out with the following parameters: - direct current or alternating current with a current density of 0.05 A / dm ² to 15 A / dm ² , - voltages of 200 V to 500 V - electrolyte temperature from 15 ° C to 45 ° C - pH of the electrolyte from 9.0 to 11.0, in particular 10.2 - conductivity of the electrolyte from 20 / cm to 40 mS / cm, in particular 33.7 mS / cm.  The method of claim 8, wherein the DC or AC is pulsed at a frequency of 10 Hz to 2000 Hz.  Method according to one of the preceding claims, wherein after the plasma-chemical anodic oxidation, a nanoscale transparent layer is deposited on the substrate by means of chemical vapor deposition.  Method according to one of the preceding claims, wherein prior to the plasma-chemical anodic oxidation, a mechanical and / or chemical pretreatment of the substrate for roughening its surface in a range of 10 .mu.m to 100 .mu.m is performed.