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Publication numberUS3640778 A
Publication typeGrant
Publication dateFeb 8, 1972
Filing dateMar 27, 1969
Priority dateMar 27, 1969
Publication numberUS 3640778 A, US 3640778A, US-A-3640778, US3640778 A, US3640778A
InventorsCharles C Mccomas, Herbert E Todd, Jules P Winfree
Original AssigneeUnited Aircraft Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Coating of titanium alloys
US 3640778 A
Abstract
Titanium and its alloys are provided with a chemically bonded coating derived from a reaction at high temperature of certain aluminum-pigmented silicone resins with the titanium substrate to form in situ by decomposition a quasi-organic complex coating which not only provides improved oxidation and corrosion resistance together with improved physical properties associated with surface-oriented characteristics, such as fatigue and creep stability, but which extends the useful operating range of a number of these alloys to temperatures of 650 DEG -900 DEG F. or higher.
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Description  (OCR text may contain errors)

O United States Patent [151 3,640,778

Winfree et al. 1 Feb. 8, 1972 [541 COATING 0F TITANIUM ALLOYS 2,743,192 4/1956 White ..117/132 x [72] Inventors: Jules P. Wlnfree, Jupiter, Fla.; Herbert E. g Todd West Hartford Conn." Charles C. m Mcc; Palm Beach Gardehs Fla 3,037,880 6/1962 Hamnk ..l [7/131 X 731 Assignce: United Aircraft Corporation, East l-lart- Primary Emminer-Ralph Kendall ford, Conn. Attorney-Richard N. James [22] Filed: Mar. 27, 1969 57 ABSTRACT PP' N05 8111071 Titanium and its alloys are provided with a chemically bonded coating derived from a reaction at high temperature of certain 521 US. Cl. ..148/6 117/132 BS 148/63 alumimm'pigmenmd Siliwne with the titanium [51] 6 7/00 strate to form in situ by decomposition a quasi-organic com- [58] field of Search ZA plex coating which not only provides improved oxidation and 1 l 4 6 corrosion resistance together with improved physical properties associated with surface-oriented characteristics, such a [56] References Cited fatigue and creep stability, but which extends the useful operating range of a number of these alloys to temperatures of UNITED STATES PATENTS 650190011; or g 2,54l,8l3 2/l95l Frisch et a]. ..1 17/46 7 Claims, 1 Drawing Figure #[47 L X/ a)? Viv-(@4760 may) Cl flfcf COATING'OF TITANIUM ALLOYS BACKGROUND OF THE INVENTION The present invention relates to the coating of titanium and the titanium alloys, particularly to the coating of those highstrength, titanium alloy components having particular utility in gas turbine engine components.

Because of their favorable strength-to-weight ratio, the titanium alloys are extensively used in certain gas turbine engines, particularly in various compressor components. Unfortunately, however, principally because of the high reactivity of titanium at elevated temperatures, the full potential of these alloys is seldom reached. While the attainment of operating temperatures in the 750 F. range would be advantageous in many instances, and this temperature level is attainable strengthwise in titanium alloy systems, the current practical limitations are nevertheless on the order of 650 F. Inasmuch as the material deficiencies are largely manifestations of a high surface reactivity, it is evident that if the full potential of the titanium alloys is to be reached, surface protection or a coating will be required.

While a number of coating techniques for the titanium alloys have been proposed, none are completely satisfactory in solving the surface reactivity problem with respect to the highly stressed alloy components in the 750900 F.-temperature range. Most often the particular prior art technique has been directed toward providing improvement to one particular property of titanium, such as galling or corrosion resistance. In most instances, however, the specific improvement is attained only at the expense of some other characteristic of the coated alloy. In the case of the metallic or ceramic coatings wherein the process results in diffusion or in an alloying reaction with the substrate metal, an intermetallic is generally formed and it is well known that the majority of the intermetallics formed with titanium are characteristically brittle.

The establishment of corrosion protection of various metals has also been provided in the past through the use of certain paints or enamels, including the pigmented silicone resins. However, as utilized, these paints, while effective in promoting corrosion resistance without the formation of brittle intermetallics or other, detrimental compounds with the basis metal, do not usually possess either the necessary adherence to the substrate or sufiicient temperature capabilities to result in substantial utility in the highly stressed high-performance applications. The silicone resins, for example, are usually limited interms of temperature to 650 F.

What is really neededin terms of surface protection for the titanium alloys is a coating whichis chemically bonded to the substratethrough a chemical reaction therewith but which does not lead-to detrimental losses in mechanical properties as a result of such interaction, particularly through the formation of brittle intermetallics, and which is capable of extending the operatingconditionsfor the titanium alloys to the 750950 F. temperaturerange.

SUMMARY OF THE INVENTION It is the primaryobject of the present invention to provide an adherentcoating for titanium and its alloys which is capable of extending the usable operating range of certain of these alloys to temperatures of 750-900 F. or higher.

It is a further object of the invention to provide asmooth oxidation and corrosion resistant coating for the titanium alloys, particularly as utilizedin connection with gas turbine engine components, which does not result in the formation of detrimental compounds or phases with the basis metal.

The above objects and other objects and advantages: are achieved by a process which results in an impervious coating which consists of the decomposition products of an aluminumpigmented silicone resin, formed in situ on thesurface to be protected, by partial oxidation thereof at high temperature and chemical reaction with the basis metal. Inasmuch as titaniurn participates in the reaction and thus contributes to its own protection in the formation of what is'thought to be an aluminum-titanium-carbon-silicon-oxygenquasi-organic polymer, the coating achieved is chemically bonded to the substrate. Furthermore, its temperature capabilities are synergistically extended to temperatures of 900 F. or higher.

The process involves coating the surface to be protected with a mixture comprising a nonreactive, polydimethyl siloxane binder and aluminum in a weight ratio of about l/1-3/ 1, preferably about 2, together with sufficient solvents for convenient application, and subsequently reacting and partially oxidizing the coating at temperatures in excess of 650 F.

In the particularly preferred process, the alloy is cleaned and deoxidiaed; coated with a mixture comprising a nonreactive polydimethyl siloxane binder and aluminum in a weight ratio of about 2; cured at 450-550 F. for at least 15 minutes; and reacted for about 1-8 hours at a temperature of 6S0-950 F. in an inert atmosphere containing about 10-40 percent oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a graph comparing the smooth fatigue capability of the titanium8 weight percent aluminum --1 percent molybdenum-l percent vanadium alloy in both an uncoated condition and as coated according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Stress corrosion of the titanium alloys is dependent upon three primary factors: the presence of corroding elements, elevated temperatures and applied or residual tensile stresses. In gas turbine engines, titanium parts are often stressed in tension above their so-called s/c level. Similarly, elevated temperatures are unavoidably present in engine operation. Therefore, under these conditions, either the corroding elements must be removed or parts must be otherwise protected before the engine operating temperatures are reached. Various schemes are and have been employed for reducing the presence of corroding elements, and various techniques, such as peening, stress relief by heat treatment and design techniques, are employed for reducing the effects of stress. However, it is apparent that metallurgical immunity to stress corrosion is far from a reality and that with todays designs utilizing available titanium alloys, coatings of some type are required for surface protection if maximum utility of these alloys is to be achieved. And, previously discussed, there is presently no coating available which affords the desired surface protection.

Concurrently with improvements in terms of stress corrosion, a truly suitable coating will also provide protection against oxidation and afford improvement with respects to those phenomena associated with surface related effects, including fatigue and creep stability, up to the limits of the titanium alloys in terms of their physical properties.

It has been found that a properly balanced formulation of particular silicone resins with appropriate metallic pigments, particularly aluminum flakes, typically with compatible diluents when applied to and cured in the right thickness on a clean, nonscaled titanium alloy surface, may be partially oxidized, employing proper controlled conditions, to provide a complex matrix of aluminum-titanium-silicon-carbonand oxygen with sufficient integrity, adherence and imperviousness to inhibit oxidative and/or corrosive action on the titanium alloy-basis metal which thereby becomes both a participant in and beneficiary of the coatings system. Synergistically, whereas the silicone paints are normally temperature limited at about 650 F., the present coating affords protection to the substrate to temperatures of 750900 F. or higher. Further, inasmuch as the titanium alloy is a participant in the reaction a chemical bond with the substrate is thus achieved.

The measured and observed beneficial effects of the coating and the apparent interaction of the basis metal as a partial contributor to its own protection, and the absence of any detectable diffusion or metallurgical (solid solution) reaction between the substrate and the coating, is in contrast to and thus not comparable to the other silicon or silicon-aluminum coatings whose bond is achieved during or by vapor deposition or by other coating techniques which depend upon metallurgical diffusion or physical bonding for adequate adhesion. The chemical ionic bonding of the present invention, as opposed to the diffused coating for titanium, does not result in any dilution of mechanical properties such as fatigue resistance.

The two titanium alloys currently receiving the major attention are those identified in the industry as MST-811 which consists of, by weight, 8 percent aluminum, 1 percent molybdenum, l percent vanadium, balance titanium; and the Ti 6-4 alloy which consists of, by weight, 6 percent aluminum, 4 percent vanadium, balance titanium.

In the most preferred coating process, the coating mixture consists of, by weight, 57.8 percent nonreactive dimethyl siloxanes, such as Dow Corning 805 and 806 silicone resins; 28.9 percent aluminum pigment, lining grade or flake aluminum; and 13.3 percent inert ingredients such as xylene. This basic formulation is available from Product Techniques Co. as PT 332.

This mixture is further diluted just prior to use with solvents such as xylene or toluene in a ratio of 4 parts pigmented resin to 1 part thinner and applied by conventional paint spraying techniques to a predetermined thickness to yield a finished coating of 0.00050.004 inch or more preferably 00015-00025 inch. it must be applied to the titanium after cleaning and deoxidation and before a substantial reoxidation The above process results in a smooth, adherent coating of good appearance and handling durability which significantly increases the useful life, strength and reliability of the highstrength titanium alloys such as MST-8l 1 in nonbeaning applications to 'stress and temperature levels appreciably higher than those currently established.

FAT GU ESTING.

. STRE S 908395. 9

Salted Creep In these tests, the time to failure at 900 F. of peened and coated specimens were established in excess of 150 hours at the stress rupture stress of 65,000 p.s.i., compared with a 150- hour life at 55,000 p.s.i. for nonpeened coated articles and less than 55,000 p.s.i. for peened only or noncoated specimens. h ssla a isswsmar sdin T blet V V coated TABLE I.-SALTED CREEP TESTING Salted creep (900 F.) Tensile (RT) Time to Elonga- Creep, failure, Strength, tion, per- Prior treatment percent hours p.s.i. cent 9. 6 0. 5 15. 9 0. 3 2. 7 3. 4 2. 5 3. 9 0.4 11.0 0. 6 12. 0 2. 2 41. 5 1. 6 29. 0 (0 14. 4 0. 9 10. 7 0. 85 10. 1 1. 25 7. 14 3. 65 1. 43 1. 5 1, 37 0.9 42 14. 2 B7 34. 8 8. 12 0. 6 8. 7 0. 3 0) 000 8. 2 4. 7 000 9. t) 2. 9 .000 5. 4 105. 6 000 2. 9 49. 4 ,000 2,12 150.0 122,300 2. 7 Do.. 55, 000 2. 20 150.0 120, 800 2.0 Glass bead peened 5N2/15N2 plus Alsilicone complex coating 85. 000 7. 86 2. 7 (0 0) D 85. 000 8. 78 2. 3 75, 000 13. 40 45. 1 75,000 10.60 32.2 000 9. 80 150.0 130, 900 2. 7 65 000 6. 85 150.0 136, 200 6. 0 1. 76 150. 0 144, 200 17. 3 1. 71 150. 0 141. 800 4. 0

r {jailed in creep.

has occurred inasmuch as a chemical reaction with the basis metal is required. Typically, the surface is chemically and/or abrasively cleaned and coated within 1 hour, or preferably 10 minutes of cleaning.

The as-sprayed coating is then allowed to air-dry usually for no less than 15 minutes or no longer than 8 hours whereat it is cured in air free of contaminants, particularly dust, moisture, halogens, and other reactive materials. The preferred curing conditions consist of 15 minutes to 2 hours at a temperature of 450550 F.

Following the curing step, partial oxidation and decomposition resulting in the formation of the finished coating is ac- Bent Beam This test entailed stressing of l-inch wideS-inch long sheet stock specimens of approximately 0.050-inch thickness in the range of plastic deformation. At 900 F. it was found that the 65 present coating provided complete protection against sea salt complished by heat treating the coating at 650950 F. for

i cident to the coating.

V Coated first-stage compressor blades subjected to 7 engine testing under 800 F. inlet condition showed that,

although the coating was abraded away from those areas subjected to severe abrasive conditions, the coating remained intact on all areas not subject to severe abrasion.

The durability of the coating is both a function of the abrasive conditions and the temperatures to which the coating is exposed. For long-term protection, exposures to temperatures up to about 750 F. would be specified although significant improvements are evident up to at least about 900 F. Even with a 750 F. temperature limitation, however, the operating range of the alloy is increased by 75-l00 F. over conventional systems, with no detrimental effects on mechanical properties, and wide temperature excursions are readily accommodated.

While the invention has been described in detail with reference to certain examples and preferred embodiments, these are intended to be illustrative only. lt will be understood that the invention is not to be limited to the exact details described, for obvious modifications will occur to those skilled in the art.

What is claimed is:

l. The method of imparting surface protection to titanium and the titanium alloys which comprises:

cleaning and deoxidizing the surface to be protected;

prior to a substantial reoxidation, coating the surface to a i ns.9 about 04 h Fi a ixtur -1.

2. The method according to claim 1 wherein:

the final heat treatment is conducted at a temperature of about 650-950 F.

3. The method according to claim 2 wherein:

the final heat treatment is conducted in an inert atmosphere containing about 10-40 volume percent oxygen.

4. The method according to claim 2 wherein:

the final heat treatment is conducted in a nitrogen atmosphere containing about 10-40 volume percent oxygen.

5. The method according to claim 4 wherein:

the weight ratio of dimethyl siloxane to aluminum is about 6. The method according to claim 4 wherein: curing is effected at a temperature of about 450-$50 F. 7. The method of coating the high-strength titanium alloys which comprises:

cleaning and deoxidizing the surface to be protected;

within about 1 hour, coating the surface to a thickness of about 00005-0004 inch with a pigmented silicone resin mixture consisting essentially of a nonreactive dimethyl siloxane and aluminum pigment in a weight ratio of about 2, and sufficient inert dispersant to permit convenient application;

within about 8 hours, curing the coating in an uncontaminated air atmosphere at 450-550 F. for at least 15 minutes;

and heat treating the coating for about 1-8 hours at a temperature of 650-950 F. in an inert atmosphere containing about 10-40 volume percent oxygen.

UNITED STATES PATENT OFFICE '(5/6 CERTIFICATE OF CORRECTION Patent No. 3,640,778 Dated uary 8, 19 7-2 Inventor) Jules P. Winfree, Herbert E. Todd, Charles C. McComas It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 5, line 24 0.005 should read 0.0005

Signed and sealed this hth day of July 1972.

(SEAL) Attest:

EIWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2541813 *Nov 8, 1947Feb 13, 1951Gen ElectricCalorizing process
US2743192 *Apr 24, 1956 He same
US2858600 *Feb 19, 1954Nov 4, 1958Gen Motors CorpSurface hardening of titanium
US2992135 *Nov 7, 1957Jul 11, 1961Crucible Steel Co AmericaReacted coating of titanium
US3037880 *May 9, 1952Jun 5, 1962Gen Motors CorpCoating of titanium and titanium alloys with aluminum and aluminum alloys
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4094750 *Oct 5, 1977Jun 13, 1978Northrop CorporationCathodic deposition of oxide coatings
US4140572 *Sep 7, 1976Feb 20, 1979General Electric CompanyProcess for selective etching of polymeric materials embodying silicones therein
US4230742 *Dec 12, 1977Oct 28, 1980Emhart Industries, Inc.Method for applying material to a substrate
US4243721 *Mar 28, 1979Jan 6, 1981Dow Corning CorporationFlexible coating resins from siloxane resins having a very low degree of organic substitution
US4243722 *Mar 28, 1979Jan 6, 1981Dow Corning CorporationNon-coloring, abrasion resistant, adherent coating for gold and silver surfaces
US4528043 *May 13, 1983Jul 9, 1985Rolls-Royce LimitedSurface oxide layer treatment
US4936927 *Dec 13, 1988Jun 26, 1990Mtu Motoren- Und Turbinen-Union Muenchen GmbhHeat treating at two different temperatures to form gradual transition between coating and alloy
US7875354Mar 28, 2008Jan 25, 2011General Electric CompanyErosions systems and components comprising the same
US7883737 *Mar 28, 2008Feb 8, 2011General Electric CompanyMethods allowing for visual inspection of coated components for erosion damage
US20090142586 *Oct 5, 2006Jun 4, 2009Hiroaki OtsukaTitanium Sheet Covered with Protective Film Superior in High Temperature Oxidation Resistance and High Temperature Salt Damage Resistance, Automobile Exhaust System Using Same, and Methods of production of Same
Classifications
U.S. Classification148/277, 428/450, 148/281, 428/447
International ClassificationB05D7/26
Cooperative ClassificationB05D3/0486, B05D2202/35, B05D7/16
European ClassificationB05D7/16