GB1566806A

GB1566806A - Masking during diffusion coating

Info

Publication number: GB1566806A
Application number: GB3865176A
Authority: GB
Original assignee: Alloy Surfaces Co Inc
Current assignee: Alloy Surfaces Co Inc
Priority date: 1975-09-19
Filing date: 1976-09-17
Publication date: 1980-05-08
Also published as: SE7610298L; FR2324757B1; CA1075980A; FR2324757A1; DE2641797A1; DE2641797C2

Description

(54) MASKING DURING DIFFUSION COATING (71) We, ALLOY SURFACES COMPANY, INC, a Corporation organised and existing under the laws of the State of Delaware, of, Wilmington, State of Delaware, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to the field of diffusioncoating metals with aluminum and chromium.

Diffusion coating of metal workpieces with aluminium and chromium is a well known technique for corrosion protection, and has wide application, in particular for parts of gas turbine engines which are exposed to particularly high temperatures (for example turbine blades). Diffusion coating is carried out at extremely high temperatures (typically 1,800 F), and generally all metal surfaces which come into contact with the coating materials at these temperatures become coated.

In many cases it is desirable to be able to form a readily frangible protective sheath over a portion of the surface of the workpiece. Such a sheath can be used to hold in place on the workpiece a localised chemical-vapourdeposition-diffusion-limiting, or masking layer (i.e. a composition capable of limiting the portion of the workpiece which becomes coated during the subsequent diffusion coating operation). It is often important to prevent the diffusion coating from adhering to certain portions of the workpiece treated. Jet engine turbine blades for example should have roots that are not coated if their mounting dimen sions are to be kept within tolerance. Also joints can be adversely affected by diffusion coating and are advantageously masked.

Shielding a root from the diffusion while exposing the balance of the blade to receiving the coating is a difficult task particularly when a quantity of the blades is treated simultaneously in the same container. The atmosphere within the container becomes a very penetrating coating medium that tends to pervade everything in the container. Shielding or masking materials may be swamped and rendered ineffective, or if very active tend to cause undesired changes in the surface of the substrate and contaminate any diffusion-coating pack used.

The invention provides a method of chemical vapor deposition diffusion coating a metal, e.g., aluminium or aluminium and chromium on a portion of the surface of a metal workpiece by first applying to that surface a localized chemical-vapor-depositiondiffusion-limiting layer and then subjecting the workpiece with that layer to vapor deposition diffusion coating, characterized in that a slurry essentially of nickel powder and a heat fugitive binder in a readily evaporated solvent is applied over the limiting layer and the solvent is permitted to evaporate before commencing the chemical vapor deposition diffusion coating and in that the diffusing metal is capable of forming a brittle sheath with the nickel.

The localized chemical-vapor-depositiondiffusion-limiting or masking layer may comprise an aluminide of nickel or cobalt together with a binder. The term 'heat-fugitive binder' as used herein, is intended to mean a binder which is driven off during the diffusioncoating process, and by the term 'consisting essentially of, the stated ingredients is meant that the composition does not contain material amounts of additorial substances having a material effect on the properties of the compositions.

The binder in the aluminide layer and the binder in the second layer may be the same, and each may preferably comprise an acrylic resin, such as polyethylmethacrylate poly (ethylethacrylate), poly (methylacrylate), poly (butylacrylate) or poly (acrylic acid). Carboxy methyl cellulose, cellulose nitrate, ethyl cellulose, polyethylene, polypropylene, polystyrene or-poly (vinyl chloride) may also be used in the binder but are less desirable. Readily volatilizable solvents such as those boiling below 100 C are preferred as solvents for the binders, such solvents are readily driven off in the subsequent diffusion-coating process.

Various aluminide-containing masking compositions are disclosed in U.S. Patent No.

380135i, and any of these may be used for the lower layer in the present invention. In particular when the workpiece is a superalloy, the diffusion limiting layer composition may preferably contain from 0.25 to 3% by weight chromium.

The following Example illusrates a preferred method of carrying out the invention.

EXAMPLE A set of uncoated jet engine blades IN-100 alloy was cleaned and their roots immersed in a first composition in the form of a stirred slurry.

The slurry comprised 50g of a powder consisting of Ni3 Al containing 2% Cr, 50g of alumina and 3g of poly (ethylmethacrylate) resin in solution in 1 00g of chloroform. The dipped blades were removed from the slurry and airaried for five minutes. The resulting blades had their roots coated with a layer ranging from about 300 to 800 milligrams per square centimeter of masking powder and resin.

The blades with the dried coating were then dipped for a few seconds in a 50% weight dispersion df powdered nickel in the same resin solution. After being withdrawn the blades were again air dried, and both coatings weighed -from about 500 to about 1200 milligrams per square centimeter. The blades were then packed in a prefired diffusion coating pack having the following composition in parts by weight: Al (minus 325 mesh) 10 Cr (about 10 micron particles size) 40 A1203 50 NH4C1 0.3 into which additional NH4 C1 was blended to bring its concentration to the designated value.

The packed assembly was heated in a retort in a gas-fired bell furnace to 1900 F where it was kept for 5 hours. Upon cooling down and opening the retort, the pack could be sucked out to the point that the blades could then be individually pulled out from the pack. The blades carried a hard shell of the originally applied layers that appeared sintered in place and did not readily crumble. It was a simple matter to remove all the blades from the pack along with all the masking mixture, leaving the remainder of the pack reusable without further separation.

The hard shell could be broken off with an easy hammer blow and the cleanly masked blades thus recovered without damaging or even endangering them. Combined coatings which together weighed only about 300 to 2000 milligrams per square centimeter were satisfactory for this purpose. Any of the other masking compositions of U.S. Patent Specification No. 3801357 can be used in the lower layer in place of the Ni3 Al.

On the other hand when using gum tragacanth or bentonite and only one masking layer, as described in Patent 3801357, the high temperatures generally cause such layer to crack so that good masking is not obtained- unless the masking coating weighs about 5 or more grams per square centimeter. Moreover such cracked coatings also disintegrate readily upon removal of the workpieces from the pack, and crumbled pieces of the masking layer wind up in the recovered pack. Such pieces must be arduously separated or the entire pack scrapped.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

WHAT WE CLAIM IS: 1. A method of chemical vapor deposition diffusion coating a metal on a portion of the surface of a metal workpiece by first applying to that surface a localized chemical-vapordeposition-diffusion-limiting layer and then subjecting the workpiece with that layer to the vapor deposition diffusion coating, characterized in that a slurry essentially of nickel powder and a heat-fugitive binder in a readily evaporated solvent is applied over the limiting layer and the solvent is permitted to evaporate before commencing the chemical vapor deposition diffusion coating and in that the diffusing metal is capable of forming a brittle sheath with the nickel.

2. A method as claimed in claim 1, wherein the organic binder is an acrylic resin.

3. A method as claimed in claim 2, wherein the acrylic resin is poly (ethylmethacrylate), poly (ethylethacrylate), poly (methylacrylate), poly (butylacrylate) or poly (acrylic acid).

4. A method as claimed in any one of the preceding claims, wherein the organic binder comprises carboxy methyl cellulose, cellulose nitrate, ethyl cellulose, polyethylene, polypropylene, polystyrene, or polyvinylchloride.

5. A method as claimed in any one of the preceding claims, wherein the solvent is an organic solvent having a boiling point of less than 100"C.

6. A method as claimed in claim 5, wherein the solvent is chloroform.

7. A method as claimed in any one of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The binder in the aluminide layer and the binder in the second layer may be the same, and each may preferably comprise an acrylic resin, such as polyethylmethacrylate poly (ethylethacrylate), poly (methylacrylate), poly (butylacrylate) or poly (acrylic acid). Carboxy methyl cellulose, cellulose nitrate, ethyl cellulose, polyethylene, polypropylene, polystyrene or-poly (vinyl chloride) may also be used in the binder but are less desirable. Readily volatilizable solvents such as those boiling below 100 C are preferred as solvents for the binders, such solvents are readily driven off in the subsequent diffusion-coating process. Various aluminide-containing masking compositions are disclosed in U.S. Patent No. 380135i, and any of these may be used for the lower layer in the present invention. In particular when the workpiece is a superalloy, the diffusion limiting layer composition may preferably contain from 0.25 to 3% by weight chromium. The following Example illusrates a preferred method of carrying out the invention. EXAMPLE A set of uncoated jet engine blades IN-100 alloy was cleaned and their roots immersed in a first composition in the form of a stirred slurry. The slurry comprised 50g of a powder consisting of Ni3 Al containing 2% Cr, 50g of alumina and 3g of poly (ethylmethacrylate) resin in solution in 1 00g of chloroform. The dipped blades were removed from the slurry and airaried for five minutes. The resulting blades had their roots coated with a layer ranging from about 300 to 800 milligrams per square centimeter of masking powder and resin. The blades with the dried coating were then dipped for a few seconds in a 50% weight dispersion df powdered nickel in the same resin solution. After being withdrawn the blades were again air dried, and both coatings weighed -from about 500 to about 1200 milligrams per square centimeter. The blades were then packed in a prefired diffusion coating pack having the following composition in parts by weight: Al (minus 325 mesh) 10 Cr (about 10 micron particles size) 40 A1203 50 NH4C1 0.3 into which additional NH4 C1 was blended to bring its concentration to the designated value. The packed assembly was heated in a retort in a gas-fired bell furnace to 1900 F where it was kept for 5 hours. Upon cooling down and opening the retort, the pack could be sucked out to the point that the blades could then be individually pulled out from the pack. The blades carried a hard shell of the originally applied layers that appeared sintered in place and did not readily crumble. It was a simple matter to remove all the blades from the pack along with all the masking mixture, leaving the remainder of the pack reusable without further separation. The hard shell could be broken off with an easy hammer blow and the cleanly masked blades thus recovered without damaging or even endangering them. Combined coatings which together weighed only about 300 to 2000 milligrams per square centimeter were satisfactory for this purpose. Any of the other masking compositions of U.S. Patent Specification No. 3801357 can be used in the lower layer in place of the Ni3 Al. On the other hand when using gum tragacanth or bentonite and only one masking layer, as described in Patent 3801357, the high temperatures generally cause such layer to crack so that good masking is not obtained- unless the masking coating weighs about 5 or more grams per square centimeter. Moreover such cracked coatings also disintegrate readily upon removal of the workpieces from the pack, and crumbled pieces of the masking layer wind up in the recovered pack. Such pieces must be arduously separated or the entire pack scrapped. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. WHAT WE CLAIM IS:

1. A method of chemical vapor deposition diffusion coating a metal on a portion of the surface of a metal workpiece by first applying to that surface a localized chemical-vapordeposition-diffusion-limiting layer and then subjecting the workpiece with that layer to the vapor deposition diffusion coating, characterized in that a slurry essentially of nickel powder and a heat-fugitive binder in a readily evaporated solvent is applied over the limiting layer and the solvent is permitted to evaporate before commencing the chemical vapor deposition diffusion coating and in that the diffusing metal is capable of forming a brittle sheath with the nickel.

6. A method as claimed in claim 5, wherein the solvent is chloroform.

7. A method as claimed in any one of the

preceding claims, wherein the localised chemical-vap or-dep osition-diffusion-limiting layer comprises an aluminide of nickel or cobalt, together with an organic binder.

8. A method as claimed in claim 7, wherein the organic binder is an acrylic resign.

9. A method as claimed in claim 8, wherein the acrylic resin is as defined in claim 3.

10. A method as claimed in any one of the preceding claims, wherein the workpiece is a superalloy, and the diffusion limiting layer composition also contains from 0.25 to 3% by weight chromium.

11. A method as claimed in any of the preceding claims, wherein the diffusing metal is aluminium.

12. A method as claimed in any one of the preceding claims, wherein the diffusing metal is chromium and aluminium.

13. A method of diffusion coating a metal workpiece substantially as hereinbefore described in the Example.