Liquid jet removal of plasma sprayed and sintered coatings

Abstract

Gas turbine engine coatings must often be removed during engine maintenance and repair. The techniques utilized to accomplish this task, machining, chemical stripping, machining followed by chemical stripping, or grit blasting, frequently result in component damage or destruction. liquid jet erosion can be utilized to remove seals, coatings, or portions thereof without damaging the engine hardware.

Description

The Government has rights in this invention pursuant to a contract awarded by the Department of the Air Force.

This application is a continuation of Ser. No. directionhelps to remove the fragments from the interaction zone thereby ensuring that they to do not interfere with the process.

FIG. 1 is one embodiment of the invention. The liquid stream (5) contacts the coating (1) at the preferred angle, approximately 45°. Additionally, the component (10) rotates such that the liquid stream (5) moves toward the smallest angle between the liquid stream (5) and the component (10) (see arrows (1A)).

The liquid stream can consist of any liquid having a viscosity between 0.25 centipoise and 5.00 centipoise at 25°C and 1 atm and which will not damage the bond coat or substrate material, including water based liquids. Higher viscosity liquids tend to present flow problems with respect to spraying the liquid at high pressures, while lower viscosity liquids can be difficult to pressurize, possibly increasing equipment costs. Water, viscosity approximately 0.95 centipoise at 25°C and 1 atm, is preferred for reasons of cost and waste disposal. Additives, such as wetting agents, or various chemicals which will degrade the coating without damaging the component, may also be useful.

A water jet pressure sufficient to remove the top coat and/or the top coat and the bond coat is required. Since pressures greater than about 60,000 psi will damage most gas turbine substrate materials, lower pressures must be used. The optimum liquid pressure ranges from about 20,000 to about 60,000 psi, with about 25,000 to about 40,000 psi preferred. The factors which determine the exact pressure required include the type of top coat and if the coating is to be removed down to the bond coat or to the substrate. (see FIG. 1A; top coat (1) and bond coat (2)). Exact pressure limits are also related to nozzle geometry and spacing, and to the specific substrate involved. In practice, the skilled artisan can readily determine the pressure which causes substrate damage and/or the pressure which causes bond coat removal and reduce this pressure to arrive at a suitable process pressure.

FIG. 2 shows the effects of varying pressures when using this invention. As the pressures decreased, from run (A) to (D), the amount of seal removed also decreases, to the point where the abradable seal/thermal barrier is removed with virtually no damage to the bond coat, (D).

This invention will be made clearer with reference to the following illustrative examples.

EXAMPLE 1

The following procedure is used to remove a plasma sprayed hard face coating, top coat and bond coat, (consisting of 20 v/o of an 80 nickel, 20 chromium alloy, balance chromium carbide) from a substrate material.

1. The coated substrate material is arranged such that relative motion can be produced between it and the water jet nozzle.

2. The water jet nozzle is placed so that the exit end of the nozzle is about 1/4 inch from the coating and the water stream contacts the coating at an angle of 45° (refer to FIG. 1).

3. The water pressure is 40,000 psi.

4. Relative motion is created between the water stream and the coating such that as the coating is removed the component advances to the next region to be removed.

5. The removal time is dependant upon the surface area of the coating. The time will range from 5 minutes to 10 minutes for typical gas turbine engine components.

EXAMPLE 2

A sintered abradable coating (consisting of approximately 65 v/o nickel, 35 v/o chrome, balance aluminum) can be removed by following the specifications set forth in Example 1, while substituting a pressure of 35,000 psi for the 40,000 psi in step 4.

This process can be used for any coating which has strength less than that of the substrate, by adjusting the pressure such that it removes the coating without bond coat damage, or the bond coat without substrate damage, allowing reuse of the bond coat and substrate or the substrate respectively.

Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

1. A method for removing a top coat from a bond coating coat adhered to a substrate, utilizing a liquid jet, said liquid jet having means for directing the liquid jet, means for creating sufficient pressure to remove the coating top coat, means to provide the relative motion between the coating top coat and the liquid jet, and means for supplying the liquid, which comprises:

a. creating sufficient pressure to remove the coating top coat;

b. providing relative motion between the coating top coat and the liquid jet;

c. supplying the liquid;

d. causing the liquid to strike the top coat, wherein the liquid striking the top coat causes top coat erosion until the bond coat is exposed;

whereby the bond coat and the substrate suffer essentially no damage and can be reused.

2. A method as in claim 1 wherein the top coat is selected from the group of plasma sprayed, flame sprayed, and sintered coatings.

3. A method as in claim 1 wherein the top coat is an abradable coating.

4. A method as in claim 1 wherein the top coat is a thermal barrier coating.

5. A method as in claim 1 wherein the top coat is an abrasive coating.

6. A method as in claim 1 wherein the coating top coat is a hard facing coating.

7. A method as in claim 1 wherein the liquid pressure is from about 20,000 psi to about 60,000 psi.

8. A method as in claim 1 using a nozzle as the means for directing the liquid flow.

9. A method as in claim 1 wherein the liquid is selected from the group of liquids consisting of all liquid which does not degrade the bond coat, and has a viscosity between about 0.25 centipoise and about 5.00 centipoise at 25°C and 1 atm.

10. A method as in claim 1 wherein the liquid is selected from the group consisting of water based liquids.

11. A method as in claim 1 wherein the liquid is essentially water.

12. A method as in claim 1 wherein the angle between the liquid stream and the top coat is between 20° and 70°; whereby the angle causes the liquid stream to clean away the coating top coat fragments.

13. A method as in claim 1 further comprising the step of removing the bond coating, wherein for removing a coating, said coating comprised of at least a top coat and a bond coat adhered to a substrate, which comprises: directing a pressurized liquid jet at a pressure above about 20,000 psi at the coating such that said liquid jet strikes said coating, thereby removing said coating from the substrate, whereby the substrate material suffers essentially no damage may be reused.

14. A method as in claim 13, wherein said top coat and said bond coat are removed simultaneously.15. A method as in claim 13, wherein said liquid jet pressure is between about 20,000 psi and about 60,000 psi.16. A method for removing a protective coating applied to a substrate which comprises: directing a liquid jet at a pressure above approximately 20,000 psi at the protective coating such that the liquid jet strikes the protective coating thereby removing the protective coating from the substrate whereby the substrate may be reused.17. A method as in claim 16, wherein the protective coating is a thermal barrier coating.18. A method as in claim 16, wherein the protective coating is an abrasive coating.19. A method as in claim 16, wherein the protective coating is an abradable seal.20. A method as in claim 16, wherein the protective coating is a hard facing.21. A method as in claim 16, wherein the liquid jet pressure is between about 20,000 psi and about 60,000 psi.22. A method as in claim 16, wherein the protective coating was applied by a pre-sintering and brazing or a partial

sintering and brazing process.23. A method as in claim 16, wherein the protective coating was applied by a partial sintering and brazing process.