Stripping of coatings Al-containing coatings

Abstract

Method for stripping a coating from a coated surface of a substrate, wherein the coating is stripped in an aqueous alkaline solution, characterized in that the method comprises following steps:—preparing the coated substrate to be decoated by providing the substrate with a strippable coating by depositing a coating comprising one or more layers, wherein one layer comprising aluminum is deposited directly on the substrate surface to be decoated and—introducting the substrate to be decoated in the aqueous alkaline solution, thereby conducting a chemical stripping of the coating from the substrate, whereas the aqueous alkaline solution comprises NaOH in a concentration in weight percentage from 30 wt. % to 50 wt. %.

Claims

1. A method for stripping a coating from a coated surface of a substrate, wherein the coating is stripped in an aqueous alkaline solution, the method comprising the following steps: preparing the coated substrate to be decoated by providing the substrate with a strippable coating by depositing a coating comprising one or more layers, wherein one layer comprising aluminum is deposited directly on the substrate surface to be decoated; and introducing the substrate to be decoated in the aqueous alkaline solution, thereby conducting a chemical stripping of the coating from the substrate, wherein the aqueous alkaline solution comprises NaOH in a concentration in weight percentage from 30 wt. % to 50 wt. %, and wherein the substrate is connected to a sacrificial anode for conducting the chemical stripping process, and the material of the sacrificial anode is selected having lower electrochemical potential than the material of the substrate to be decoated, or having lower electrochemical potential than material constituents contained in the substrate to be decoated that can be dissolved during chemical stripping.

2. The method according to claim 1, wherein the layer deposited directly on the substrate surface to be decoated is selected from the group consisting of: a metallic interlayer layer, a wurtzite aluminum nitride layer, a layer comprising wurtzite aluminum nitride, and an aluminum oxide layer.

3. The method according to claim 1, wherein the chemical stripping is conducted by maintaining the aqueous alkaline solution at a temperature corresponding to a value in a range from 80° C. to 160° C., the temperature range including border values 80° C. and 160° C., respectively.

4. The method according to claim 3, wherein the temperature of the aqueous alkaline solution is a value in a range from 100° C. to 140° C., the range including border values 100° C. and 140° C., respectively.

5. The method according to claim 1, wherein the concentration of NaOH in the aqueous alkaline solution is in a concentration range from 40 wt. % to 50 wt. %, the concentration range including border values 40 wt. % and 50 wt. %, respectively.

6. The method according to claim 1, wherein the sacrificial anode is made of iron or is made of a material comprising iron.

7. The method according to claim 6, wherein the substrate is made of cemented carbide having a cobalt binder phase or the substrate to be decoated is made of a material comprising cemented carbide having a cobalt binder phase.

8. The method according to claim 1, wherein the substrate to be decoated is made of cemented carbide or the substrate is made of a material comprising cemented carbide.

9. A method for stripping a coating from a coated surface of a substrate, wherein the coating is stripped in an aqueous alkaline solution, the method comprising the following steps: preparing the coated substrate to be decoated by providing the substrate with a strippable coating by depositing a coating comprising one or more layers, wherein one layer comprising aluminum is deposited directly on the substrate surface to be decoated using a physical vapor deposition process, and before conducting the deposition process a blasting media comprising alumina is used for blasting one or more cathodes that are used as a coating material source for depositing said layer directly on the substrate; and introducing the substrate to be decoated in the aqueous alkaline solution, thereby conducting a chemical stripping of the coating from the substrate, wherein the aqueous alkaline solution comprises NaOH in a concentration in weight percentage from 30 wt. % to 50 wt. %.

Description

(1) Further aspects of the invention will be illustrated and explained in more detail for improving understanding of the present invention. Following figures should help to illustrate and understand the present invention.

(2) Figure captions:

(3) FIG. 1: Scanning electron micrograph (SEM) of the surface of polished cemented carbide substrate, WC with 6 wt. % Co, prior to applying coating.

(4) FIG. 2: Scanning electron micrograph (SEM) of the surface of polished cemented carbide substrate, after 10 minutes immersion in stripping media according to state of the art: 4 wt. % KMnO4, 8 wt. % NaOH. The surface is strongly attacked, WC grains are dissolved, Co-matrix is remained.

(5) FIG. 3: Scanning electron micrograph (SEM) of the surface of polished cemented carbide substrate, after 5 hours immersion in stripping media according to the present invention: 50 wt. % NaOH at 140° C. The substrate surface is intact.

(6) FIG. 4: Schematic drawing of the stripping process construction including sacrificial anode. 1: AlCrN coating, 2: Interlayer, 3: WC/Co-substrate (cemented carbide substrate), 4: sacrificial anode made of iron having electrical connection with the substrate to be decoated according to the present invention, 5: Stripping solution comprising NaOH according to the present invention, 6: Oxidation layer formed of iron oxide produced by oxidation of the sacrificial anode.

(7) In order to explain the present invention in more detail, some examples will be described. The described inventive examples should not be understood as a limitation of the invention but only as showcases of possible and for most cases preferred embodiments of the invention.

Example 1 Comparative Example

Stripping Process According to State of the Art

(8) State of the art PVD coatings, in particular AlCrN deposited from targets with composition Al.sub.70Cr.sub.30 in atomic percentage, are stripped using a solution of 4 wt. % KMnO.sub.4 and 8 wt. % NaOH. The coated substrates are left in the stripping solution for 20 minutes until complete dissolution of the coating. The substrate surface is attacked and consequently looks after decoating process strongly damaged and rough.

Example 2 Inventive Example

(9) An inventive process was conducted comprising following steps:

(10) I) Coating preparation step—before the PVD coating procedure, the surface of AlCr-targets with composition Al.sub.70Cr.sub.30 in atomic percentage, which were used in the PVD process step were treated previously by sandblasting. The sandblasting media consists of Al.sub.2O.sub.3 of grit size 0.25-0.5 mm. Blasting pressure of 1-5 bars, typically 4 bars was used, and the basting time was about up to 30 seconds to cover the surface are of one target.

(11) II) Application of a PVD coating of AlCrN using the deposition targets prepared in step I. N.sub.2 is used as process gas.

(12) III) Chemical stripping of the coating by introducing the coated substrates in a 50 wt % NaOH solution at 130° C. After 4 hours, the coating (3 μm) was completely removed from the substrates. The substrates did not show any damage or degradation of the substrate surface.

Example 3 Inventive Example

(13) One more inventive process was conducted comprising following steps: I) At the beginning of the coating process, a metallic interlayer was deposited by using PVD techniques, wherein the metallic interlayer was deposited directly on the substrates by using Al70Cr30-targets as coating material source and Ar gas flow as process gas in the coating chamber, typically 300 sccm for 20 s up to 5 min. The thickness of the metallic interlayer ranged between 20 nm to 460 nm.

(14) II) A coating layer of AlCrN was deposited on the metallic interlayer by using N.sub.2 gas flow as process gas (in this case also called reactive gas because of the reaction with AlCr material from the targets of composition Al70Cr30) for forming an AlCrN coating layer on the interlayer deposited on the substrates being coated.

(15) III) Chemical stripping of the coating by introducing the coated substrates in a 50 wt % NaOH solution at 130° C. After 1.5 hours, the coating was completely removed from the substrates. The substrates did not show any damage or degradation of the substrate surface.

Example 4 Inventive Example

(16) One more inventive process was conducted comprising following steps:

(17) I) At the beginning of the coating process, an interlayer containing wurtzite-AlN was deposited by using PVD techniques from AlCr targets having element composition Al90Cr10 in atomic percentage, which were used as coating material source in a coating chamber comprising N.sub.2 gas flow as process gas (in this case also called reactive gas because of the reaction with AlCr material from the targets) for forming AlCrN coating layers containing wurtzite-AlN.

(18) II) A coating layer of AlCrN was deposited on the interlayer containing wurtzite-AlN by using PVD techniques from AlCr targets having element composition Al70Cr30 in atomic percentage, which were used as coating material source in a coating chamber comprising N.sub.2 gas flow as process gas for forming an AlCrN coating layer on the interlayer deposited on the substrates being coated.

(19) III) Chemical stripping of the coating by introducing the coated substrates in a 50 wt % NaOH solution at 130° C. After 1.5 hours, the coating was completely removed from the substrates. The substrates did not show any damage or degradation of the substrate surface.

Example 5 Inventive Example

(20) One more inventive process was conducted comprising following steps:

(21) I) At the beginning of the coating process, a metallic interlayer was deposited by using PVD techniques, wherein the metallic interlayer was deposited directly on the substrates by using AlCr-targets of composition Al70Cr30 in atomic percentage as coating material source and Ar gas flow as process gas in the coating chamber as described in Example 3.

(22) II) A coating layer of AlCrN was deposited on the metallic interlayer by using N.sub.2 gas flow as process gas (in this case also called reactive gas because of the reaction with AlCr material from the targets—same targets as used in step I) for forming an AlCrN coating layer on the interlayer deposited on the substrates being coated.

(23) III) Exposing the samples to an acid (HNO.sub.3, H.sub.2SO.sub.4, HCl) for a kinetically controlled reaction. In particular, one sample (substrate coated as indicated in previous steps I and II) was exposed to HCl for 2 minutes. Another sample (also a substrate coated as indicated in previous steps I and II) was exposed to H.sub.2SO.sub.4 for 2 minutes.

(24) IV) Chemical stripping of the coating by introducing the coated substrates in a 50 wt % NaOH solution at temperature of 90° C. (40° C. lower compared to Example 3). After 3 hours, the coating was completely removed from both substrates.

Examples 6, 7, 8, 9 and 10

(25) These examples have all the same steps I and II as described below but differentiate each one from each other in the conduction of the chemical stripping process, i.e. in following step III:

(26) I) Before initiating coating step, the surface of AlCr-targets to be used afterwards in the deposition were treated by sandblasting. The sandblasting media consisting of Al.sub.2O.sub.3 of grit size 0.25-0.5 mm. Blasting pressure of 1-5 bars, typically 3 bars was used, and the blasting time was about up to 30 seconds to cover the surface are of one target.

(27) II) Conducting coating process, wherein a coating layer of AlCrN was deposited on the substrate surface to be coated by using PVD techniques from AlCr targets having element composition Al70Cr30 in atomic percentage, which were used as coating material source in a coating chamber comprising N.sub.2 gas flow as process gas for forming an AlCrN coating layer on the substrates being coated

(28) III) 6 different samples (substrates coated by following the steps I and II as described directly above) undergo chemical stripping of the coating as following:

Step III of Example 6

(29) IIIc. LiOH was used as base in a saturated aqueous solution having LiOH concentration of 13 wt. %. Partial stripping was observed after 1 h, slower stripping rate compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

Step III of Example 7

(30) IIIb. KOH was used as saturated aqueous solution having KOH concentration of 50 wt. %. Partial stripping was observed after 1.5 h, slower stripping rate compared to compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

Step III of Example 8

(31) IIIc CsOH was used in an aqueous solution having CsOH concentration of 75 wt. %. Partial stripping was observed after 3 h, slower stripping rate compared to compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

Step III of Example 9

(32) IIId. Base/salt mixture of NaOH:KOH in molar rate 1:1 was used in a saturated an aqueous solution having NaOH concentration of 50 wt. % and KOH concentration of 50 wt. %. Partial stripping was observed after 1 h, slower stripping rate compared to compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

Step III of Example 10

(33) IIIe. Base/salt mixture of NaOH:NaCl in molar rate 1:1 was used in a saturated aqueous solution having NaOH concentration of 50 wt. % and NaCl concentration of 25 wt. %. Partial stripping was observed after 20 min, slower stripping rate compared to compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

(34) IIIc. Base/salt mixture of KOH:NaCl in molar rate 1:1 was used in a saturated aqueous solution having KOH concentration of 50 wt. % and NaCl concentration of 25 wt. %. Partial stripping was observed, slower stripping rate compared to compared to chemical stripping processes in which a NaOH 50 wt. % concentration was used.

(35) Methods according to the present invention have been also successfully used for decoating of substrates coated with many different kind of coatings, e.g. TiAlN-, TiAlSiN-, TiAIN-based and TiAlSiN-based coatings, amongst others.

(36) The aqueous solution can be agitated or the substrates being decoated can be moved during chemical stripping process. The method according to the present invention can also comprise further pre-treatment or post-treatment steps, e.g. rinsing, cleaning, ultrasound bad treatments, drying and heat treatments, amongst others.

(37) Independent protection, too, is claimed for a system of tools with a renewable coating characterized in that the system is constituted by coated tools that carry—directly on its surface to be stripped for recoating— a coating comprising aluminium and that is constituted and by a stripping agent in the shape of an aqueous alkaline solution that comprises NaOH in a concentration in weight percentage from 30 wt. % to 50 wt. %.