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 present invention discloses a method for removing a top coating from a bond coating adhered to a substrate according to the precharacterizing portion of claim 1, and a method for removing a bond coating according to the precharacterizing features of claim 13. This invention relates to the removal of coating materials, and specifically to the removal of abradable, wear resistant, and thermal barrier coating materials which have been applied by either sintering powder or fibers, or by plasma spraying, utilizing liquid jet erosion. The DE-U-8 907 917.5 describes a method for removing a coating from a substrate in utilizing a liquid jet having means for directing the liquid jet, means for creating a sufficient pressure to remove the coating, means to provide the relative motion between the coating and the liquid jet and means for supplying the liquid. Various types coatings and sintered materials are used in numerous applications, such as in gas turbine engines to increase efficiency and/or protect components from heat and wear. Types of materials include thermal barrier coatings, abrasive coatings, abradable seals, and hard facing; hereafter referred to as coatings. Since excessive blade/case clearances and disc/vane clearances within turbine engines allow the escape of gases which decreases engine efficiency, an abradable seal can be applied to minimize the clearances between the rotating and the stationary components. Thermal barrier coatings can be utilized to provide protection against high temperatures, while abrasive coatings can be used to prevent detrimental rub interactions and hard facing can be used to reduce wear. Some coatings are applied by plasma or flame spraying; introducing particles (usually powders) into a hot gas stream or flame (respectively) which causes the particles to splat onto the substrate surface where they adhere and build up as a coating. Application of particles (i.e. AB-1) or short wires (i.e. Feltmetal™) onto a substrate; by pre-sintering or partial sintering and then brazing, can be used to produce abradable coatings comprised of bonded particles, wires, or powders and void spaces; while bond coats can be produced by plasma spraying or vapor deposition. Bond coats are usually used in plasma spray and vapor deposition applications; a bond coat being a layer of metallic composition applied to the substrate before the coating is applied. US-A-. 3,542,530, 3,676,085, 3,754,903, 3,879,831, 3,928,026, and 4,704,332, describe various coatings, while US-A-. 3,413,136, 4,055,705, and 4,321,311 describe application techniques. A common characteristic of these types of coatings is that the coating strength (cohesive strength) is relatively low; plasma sprayed or partially sintered particles are not well bonded to each other and there is usually porosity present. The strength of the coating is less than that of the substrate. During engine maintenance, these coatings must frequently be removed; a process difficult to reliably preform and which frequently results in substrate damage. Various techniques have been employed for the removal of coatings: machining, chemical stripping, machining followed by chemical stripping (see for example US-A-.4,339,282, and 4,425,185; ),and grit blasting. For example, machining followed by chemical stripping requires that the component be held stationary while a machining tool removes the majority of the coating. A chemical solution, usually either a very strong acid or base, is then applied to the coating surface to disintegrate the remaining coating material. This technique requires extreme precision; without proper hardware alignment during machining damage to the substrate material occurs, while the chemical solution used tends to attack the substrate material. This process is also time consuming and labor intensive. Additionally, the chemical step, can produce hazardous waste. The individual processes of chemical stripping and machining also have the above described problems. Another commonly used method, abrasive or grit blasting, also often results in damaged or destroyed components. This process consists of projecting abrasive particles in a compressed air stream against the coating. Since this technique requires immediate termination upon substrate exposure to prevent damage, it requires skilled operators. Liquid jets above 68.95 MPa (10,000 psi), to the best of our knowledge, have not been utilized in the removal of coatings. Relatively low pressure liquid jets, 13.79 to 20.69 MPa (2,000 to 3,000 psi), have been applied in areas such as: cleaning applications, nuclear contamination removal, concrete scarifying, and barnacle and hull fouling removal, but not in an inorganic coating removal process. Accordingly, an objective of this invention is to provide a convenient, cost effective, environmentally safe technique of removing coatings. The method of the present invention for removing a top coating from a bond coating is defined in the characterizing portion of claim 1 and the method for removing the bond coating is defined in the characterizing portion of claim 13. The present invention involves the removal of coatings utilizing a liquid jet erosion process. The liquid jet, while striking the coating at an angle, traverses the region, removing the coating. Depending on the liquid pressure, the liquid stream erodes the abradable seal/thermal barrier with virtually no damage to the bond coat (if present), or can remove both the abradable seal/thermal barrier and bond coat simultaneously without substrate damage. The invention process can be used to remove plasma sprayed and sintered coatings whose cohesive strength is significantly less than that of the substrate. The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings. Fig. 1 is a basic embodiment of this invention. Fig. 1A is a cross-section of Fig. 1 which reveals the various layers of the coating. Fig. 2 shows the results of utilizing a liquid jet removal process at varying pressures. The removal of coatings using current techniques is a difficult, inexact process. It requires skilled technicians, a substantial amount of time, expensive equipment, and frequently, the component is destroyed. The removal of the coating, bond coat, or both without damage to the substrate material can be achieved with a liquid jet erosion technique; making it a viable alternative to the prior art. As previously mentioned, this invention uses a liquid jet erosion process to remove coatings. Critical parameters (see Fig. 1) include the nozzle distance from the coating, and the liquid pressure. Depending on equipment and pressure constraints, the nozzle can be placed up to approximately 15.2 or even 30.5 cm (6 or even 12 inches) from the coating surface, however, lesser distances are preferred, with 6.35 to 19.0 mm (1/4 to 3/4 inch) especially preferred. The angle between the liquid contact and the coating is a matter of preference. An angle of between 20° to 90° can be used, with an angle of between 30° and 90° preferred, and an angle of about 45° especially preferred (see Fig. 1). The angle, not a critical parameter, causes the liquid to remove the coating fragments from the region where the jet impacts the coating. The direction of rotation effects the fragment location post-removal. It is preferred to rotate the component such that the motion is toward the smallest angle formed between the liquid stream and the component. Although this is merely a matter of preference, this rotation directions helps to remove the fragments from the interaction zone thereby ensuring that they to do not interfere with the process. The liquid stream can consist of any liquid having a viscosity between 0.25 mPa s (centipoise)and 5.00 mPa s (centipoise)at 25°C and 101.3 kPa (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 mPa s (centipoise) at 25°C and 101.3 kPa (1 atm), is preferred for reasons of cost and waste disposal. Although 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 coating and/or the coating and the bond coat is required. Since pressures greater than about 413.7 MPa (60,000 psi)will damage most gas turbine substrate materials, lower pressures must be used. The optimum liquid Pressure ranges from about 137.9 MPa(20,000) to about 413.7 MPa (60,000 psi), with about 172.4 MPa (25,000) to about 275.8 MPa (40,000 psi) preferred. The factors which determine the exact pressure required include the type of coating and if the coating is to be removed down to the bond coat or to the substrate. (see Fig. 1A; coating (1) and bond coat (2)). Exact pressures 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, coating 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 6.35 mm (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 275.8 MPa (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 241.3 MPa (35,000 psi) for the 275.8 MPa (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. 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 scope of the claimed invention.

Claims: A method for removing a top coating from an underlying bond coating adhered to a substrate, utilizing a liquid jet, said liquid jet having means for directing the liquid, means for creating sufficient pressure to remove the coating, means to provide the relative motion between the coating and the liquid jet, and means for supplying the liquid, which comprises: a) supplying the liquid at a sufficient pressure to remove the coating; b) providing relative motion between the coating and the liquid jet, characterized in that the liquid jet strikes the top coating at a pressure sufficient to remove the top coating until the bond coating is exposed, whereby the bond coating and the substrate material suffer essentially no damage and can be reused. A method as in claim 1 characterized in that the top coat is selected from the group of plasma sprayed, flame sprayed, and sintered coatings. A method as in claim 1 characterized in that the top coat is an abradable material. A method as in claim 1 characterized, in that the top coating is a thermal barrier. A method as in claim 1 characterized in that the top coating is an abrasive material. A method as in claim 1 characterized in that the top coating is a hard facing material. A method as in claim 1 characterized in that the liquid pressure is from 137.9 MPa (20,000 psi) to 413.7 MPa (60,000 psi). A method as in claim 1 characterized in using a nozzle as the means for directing the liquid flow. A method as in claim 1 characterized in that the liquid is selected from the group of liquids consisting of all liquid which does not degrade the bond coating, and has a viscosity between 0.25 mPa s (centipoise) and 5.00 mPa s (centipoise) at 25°C and 101,3 kPa (1 atm). A method as in claim 1 characterized in that the liquid is selected from the group consisting of water based liquids. A method as in claim 1 characterized in that the liquid is essentially water. A method as in claim 1 characterized in that the angle between the liquid stream and the top coating is between 20° and 70°; whereby the angle causes the liquid stream to clean away the coating fragments. A method for removing a bond coating utilizing a liquid jet, said liquid jet having means for directing the liquid jet, means for creating sufficient pressure to remove the bond coating, means to provide the relative motion between the bond coating and the liquid jet, and means for supplying the liquid jet, which comprises: a) supplying the liquid jet at a sufficient pressure to remove the bond coating; b) providing relative motion between the bond coating and the liquid jet, characterized in that the liquid jet strikes the bond coating at a pressure of above about 137.9 MPa to remove the bond coating until the substrate material is exposed whereby the substrate material suffers essentially no damage. A method as in claim 13 characterized in that the bond coat is produced using a method selected from the group consisting of plasma spraying and vapor deposition. A method as in claim 13 characterized in that the liquid pressure is from 137.9 MPa (20,000 psi) to 413.7 MPa (60,000 psi). A method as in claim 13 characterized in using a nozzle as the means for directing the liquid stream. A method as in claim 13 characterized in that the liquid is selected from the group consisting of all liquids which do not damage the substrate material, and which have a viscosity between 0.25 mPa s (centipoise) and 5.00 mPa s (centipoise) at 25°C and 101.3 kPa (1 atm). A method as in claim 13 characterized in that the liquid is selected from the group of water base liquids. A method as in claim 13 characterized in that the liquid is water. A method as in claim 13 characterized in that the angle between the liquid jet and the bond coating is between 20° and 70°; whereby the angle causes the liquid jet to clean away the bond coat fragments. An article obtainable by the method of claim 1 said article having a bond coating whereby the bond coating is exposed after the removal of the abradable coating, wherein the substrate and the bond coating are essentially undamaged and reusable. An article obtainable by the method of claim 1 said article having a bond coating, whereby the bond coating is exposed after the removal of the thermal barrier coating, wherein the substrate and the bond coating are essentially undamaged and reusable. An article obtainable by the method of claim 1, said article having a bond coating, whereby the bond coating is exposed after the removal of the abrasive coating, wherein the substrate and the bond coating are essentially undamaged and reusable. An article obtainable by the method of claim 1, said article having a bond coating whereby the bond coating is exposed after the removal of the hard face coating, wherein the substrate and the bond coating are essentially undamaged and reusable.

Inventors: APPLEBY JOHN W JR, BARRINGER HERBERT R, MCCOMAS CHARLES C, PATRY MICHAEL J, SILEO GERARD A, APPLEBY, JOHN W., JR., BARRINGER, HERBERT R., MCCOMAS, CHARLES C., PATRY, MICHAEL J., SILEO, GERARD A.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1990
Main classification: B08B9
IPC codes: B26F3, B08B3, B08B9, C23F4, B23P17