US3019516A

US3019516A - Method of forming a protective coating on molybdenum

Info

Publication number: US3019516A
Application number: US813034A
Authority: US
Inventors: James C Holzwarth; Jr James R Hornaday; Charles W Vigor
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1959-05-13
Filing date: 1959-05-13
Publication date: 1962-02-06
Anticipated expiration: 1979-02-06

Description

3,019,516 Patented Feb. 6, 1962 3,019,516 METHOD OF FORMING A PROTECTIVE COATING N MOLYBDENUM James C. Holzwarth, Birmingham, James R. Hornaday, In, Royal Oak, and Charles W. Vigor, East Detroit, Mich, assignors to General Motors (Iorporation, Detroit, Mich, a corporation of Delaware I No Drawing. Filed May 13, 1950, Ser. No. 813,034

12 Claims. (Cl. 29-198) This invention relates to a protective coating for molybdenum and molybdenum base alloys. It pertains particularly to a process for providing a molybdenum base metal turbine blade or similar article with a composite coating which possesses good high temperature oxidation resistance and excellent ductility with respect to impact indentation and creep deformation.

The nickel base alloy and cobalt base alloy blades commonly used today in gas turbine engines normally have maximum service temperatures of approximately 1800 F. to 1900 F. This limitation necessarily restricts the performance and efficiency of these engines. Molybdenum has a satisfactory high melting temperature and suflicient potential availability to warrant investigation as a high temperature turbine blade material. However, it possesses poor oxidation resistance at temperatures of 1200 F. or above. Therefore, molybdenum is unsatisfactory for use in turbine blades which necessarily are exposed to extremely hot oxidizing gases. During recent years attempts have been made to correct this deficiency by adding small amounts of various alloying elements to molybdenum. However, such attempts have been unsuccessful since the resultant products still do not possess adequate oxidation resistance at 2000 F. and above, the temperature conditions under consideration in the present invention.

' Likewise, considerable effort has been expended in the development of coatings for molybdenum in order to permit this metal to be used at elevated temperatures under oxidizing conditions. In the past, for example, nickelchromium alloys have been applied to molybdenum sheets by roll cladding, and an alloy layer of this type protects molybdenum from oxidizing environments at elevated temperatures. Prior to the present invention, however, such processes for coating molybdenum usually involved heating the base metal above its recrystallization temperature, thereby producing embrittlement. Heretofore molybdenum also has been coated with chromium-nickelboron and chromium-nickel-silicon-boron alloys. However, the utility of these coatings is limited by their relatively low melting points and the tendency for the nickel to diffuse into and embrittle the molybdenum.

A principal object of the present invention, therefore, is to provide a protective coating for molybdenum and molybdenum base alloys winch is oxidation-resistant in air at temperatures as high as 2200 F. In addition, if gas turbine blades or similar articles formed of molybdenum base alloys are to be coated, the coating must possess sufiicient ductility to resist mechanical impact, creep deformation and thermal shock. A further object of the invention, therefore, is to provide a process for producing such a coated article which possesses a combination of these desirable properties to an outstanding degree.

These and other objects are attained with this invention by successively coating molybdenum or molybdenum base alloys with layers of chromium, nickel-chromium alloy and nickel-chromium-boron alloy. The present process produces a satisfactory protective coating on finish machined molybdenum articles of complex shape without heating the base metal above its recrystallization temperature. Of course, it will be understood that the term molybdenum, as used herein, is intended to also encompass molybdenum base alloys which require protection against oxidation at elevated temperatures.

More specifically, the coating employed in accordance with this invention is a composite which includes an electroplated chromium layer and an overlay of an alloy comprising about 15 to 25% chromium and the balance substantially all nickel. However, alloys of this type having a chromium content as high as 50% also are useful for this purpose. An nickel-20% chromium alloy is generally preferred. The chromium electroplate functions as a barrier to prevent the nickel from diffusing into the molybdenum since such diffusion produces rapid and serious embrittlement of the base metal. The chromium preferably is deposited from a bath under operating conditions which produce a soft, crack-free chromium deposit.

A dry hydrogen annealing treatment is employed after the chromium electrolplating process to further soften the plate and establish a difiusion bond to the molybdenum. This heat treatment also serves as an inspection procedure for determining the degree of adherence of the chromium plate to the molybdenum. A one-hour treatment at 2000 F. appears to be optimum, although the temperature and time may vary appreciably depending on the particular molybdenum base metal being treated. 1700 F. is considered the minimum diffusion temperature, but annealing at that temperature requires too long a time to be commerically practical. The maximum permissible temperature is that immediately below the recrystallization point of the'molybdenum or molybdenum base alloy article being treated. Hence, the maximum temperature mayvary from 1800 F. to 3000 F., but the recrystallization temperatures of the typical molybdenum base alloys under consideration range from 2200 F. to 2600 R, depending on the alloy composition and the prior working history of the article. At the practical maximum heat treating temperature of 2200 F., the heat treating time is reduced to approximately one-half hour.

It is recognized that the chromium electroplate is, in itself, a good oxidation-resistant coating for molybdenum. As is well known, however, chromium becomes embrittled as a result of high temperature exposure in air, principally because of nitrogen pickup, and hence a protective overlay is required for high temperature applications in oxidizing or air atmospheres.

We have obtained excellent results with a chromium plate having a thickness of about 1 mil, although the chromium generally functions as a satisfactory barrier layer when its thickness ranges from about 0.5 mil to 4 mils. However, normally it is not particularly advantageous to use a chromium plate thicker than 2 mils. When a relatively thin chromium layer is applied, it is particularly important to insure that this layer is soft and crack-free.

The nickel-chromium layer may be built up over the chromium plate by flame spraying a'powdered pro-alloy of about 80% nickel and 20% chromium, for example. A highly satisfactory layer also is formed by flame spraying to a controlled weight per unit area a powdered metal mixture of an alloy of 80% nickel and 20% chromium to which has been added a nickel-chromium-boron alloy in an amount equal to approximately 5% to 15% of the nickel-chromium alloy. The ternary alloy facilitates sintering in a subsequent heat treatment by melting and bonding the nickel-chromium alloy particles together, thereby permitting the use of a lower sintering temperature. While it eliminates tendencies toward shrink cracking, it does not reduce the porosity of the nickel-chromium alloy.

The nickel-chromium-boron alloy preferably comprises about to 17% chromium, 2% to 4% boron and the balance substantially all nickel. However, satisfactory results can be obtained with a chromium content as high as 20% and a boron range of approximately 0.5% to 6%. Of course, small amounts of the other minor constituents may be present in the alloy without detracting from its utility. For example, up to about 5% silicon and 1% carbon further lower the melting point of the nickelchromium-boron alloy, but the use of a large amount of silicon is undesirable because silicon, as compared with boron and carbon, will not diffuse out of the alloy to as great a degree during heat treatment. Likewise, a high carbon content should be avoided since it tends to form brittle carbides in the nickel-chromium alloy. Iron also may be in the alloy, and 5% or so iron is frequently found in alloys of this type. Other elements, such as copper, cobalt, tungsten and molybdenum, may be present in small quantities in the sprayed coating layer without adversely affecting it. An example of a suitable commercially available nickel-chromium-boron alloy is one consisting of approximately 13.5% chromium, 3.5% boron, 4.5% silicon, 4.5% iron, 0.8% carbon and the balance nickel.

We have found that generally the nickel-chromium alloy should be applied in an amount to form a layer weighing about 0.1 to 0.15 gram per square centimeter, 0.125 gram per square centimeter being considered optimum at present. In the as-sprayed or sintered condition a nickelchromium alloy layer of this weight will have a thickness of approximately 5 to mils, depending on the density of the sprayed mass. A metal powder of -60 to +200 mesh size is preferred.

After the nickel-chromium layer has been formed on the surface of the chromium electroplate, the coated article is subjected to a second dry hydrogen heat treatment cycle which reduces any metal oxides generated and entrapped by the flame spraying. This procedure produces an overlay of a clean but fairly porous metal network somewhat resembling a powdered metallurgy sintered compact. The heat treatment sinters the nickelchromium particles, but the sprayed layer remains essentially porous. The high temperatures (i.e., approximately 2400 F.) necessary for complete fusion of the particles are to be avoided since exposure to such temperatures recrystallizes the base molybdenum. Of course, the temperature should be high enough to reduce any metal oxides present and to sinter the nickel-chromium layer to a sufiicient extent to prevent chipping of the layer during handling. Likewise, the coating must not become so 'hot as to close the pores and shrink and craze the coating. A temperature range similar to the one applicable in the initial annealing treatment is generally appropriate. However, it is advantageous to begin this second heat treatment at a relatively low temperature, such as 1700 F., to permit presintering and eliminate excessive shrinkage and cracking of the nickelchromium alloy coating. 7

The porous outer layer thus formed is subsequently impregnated with a nickel-chromium boron alloy of the aforementioned type. This may be accomplished by flame spraying a powdered alloy over the sintered nickel-chromium alloy in an amount equal to about 0.1 to 0.15 gram per square centimeter. In general, the weights per unit area of the nickel-chromium alloy and the nickel-chromium-boron alloy are preferably about equal. In the case of the latter, the thickness of the sprayed overlay will be about 3 to 10 mils when this amount of metal powder is used. A nickel-chromiumboron alloy having a particle size of about to +200 mesh produces excellent results.

The coated article is thereafter subjected to a'brief. high temperature heat treatment in a dry hydrogen atmosphere to cause the nickel-chromium-boron alloy to 'melt and penetrate into the pores "of the matrix nickelchromium alloy. It is desirable to heat the coated article as quickly as possible to a temperature of 2200 F. to 2250" F. and to maintain this temperature until the pores of the nickel-chromium alloy are filled. This normally requires ten to twenty minutes. The rapid high temperature heating is advantageous since it causes the ternary alloy to infiltrate the nickel-chromium alloy matrix before the boron and silicon, if any is present, are completely diffused out of the overlay. Such premature total diffusion would raise the melting point of the nickel-chromiummoron alloy excessively before the pores of the matrix were filled. Above 2250 F. the alloy becomes too fluid and tends to erode away and drip off the molybdenum base metal article being coated. The minimum heat treating temperature is the flow point of the nickel-chromium-boron alloy (i.e., approximately 1800 F.), but the alloy will not flow satisfactorily in the short time desired unless the temperature is about 2100 F. or above.

A programmed diffusion treatment cycle is next employed to reduce the local boron concentration gradients in the coating by solid state diffusion into the nickelchromium alloy. A substantially dry hydrogen atmosphere is again employed with annealing temperatures of 2000 F. to 2250 F. being appropriate. While this diffusion treatment temperature may be appreciably lower than the temperature initially used to melt the nickel-chromium-boron alloy overlay, temperatures as low as 1800 F. require heat treatment times which are too long to be practical. As a result of this heat treatment sequence, the outer layer of nickel-chromium alloy becomes homogeneous and has a melting point considerably above the heat treatment temperature. The total composite protective layer, including the chromium plate, preferably has a thickness of about 7 or 8 mils, although this layer may vary from 5 to 20 mils and still be effective.

While the'use of a dry hydrogen atmosphere in the coating eifectively protects molybdenum for more than 1,000 hours at a temperature of 2000 F. in air and for approximately 800 hours at a temperature of 2200 F. Moreover, the ductility of the above-described coating on molybdenum is highly satisfactory with respect to ballistic impact resistance and creep deformation. Our tests show that coated molybdenum stress-rupture specimens can undergo appreciable elongation without cracking under high temperature creep loads in air. For example, the coating has withstood at least 5% creep elongation at 1800 F. and 10% creep elongation at 2000" F. in'air. When the impact resistance of the coating was evaluated by a ballistic impact test, the coating was shown to be capable of indentation without spalling or cracking at temperatures of 1800 F. .to 2000 F. Thermal shock is equal'to'the best of any other fused metal coating, cladding or electrodeposited coating.

Attention should be given to the geometry of th article to be coated and, in general, it should be free of sharp edges with fillet radii being as large as practicable.

Of course, all' surfaces must be accessible for metal' spraying and uniform electroplating. With these generalities in mind, the following specific example illustrates a preferred embodiment of the coating procedure hereinbeforedescribed. In general, we have found itdesirable to initially grit blast the surfaces of the mqlyb- Following this surface preparation of the molybdenum, chromium is electroplated on the article to a depth of about 1 to 1.5 mils. Before the actual electrodeposition step, the article is advantageously subjected to an abrasive scrub followed by a water rinse, an acid etch and a second water rinse. A 50% hydrochloric acid solution may be effectively used as the etchant.

A useful chromium plating bath is one containing approximately 33 ounces per gallon of chromic acid, 0.2 ounce per gallon of sulphuric acid and 1.57 ounces per gallon of an additive designed to promote the formation of a soft and crack-free plate. Additive CPA 1800, currently sold by the Diamond Alkali Company of Cleveland, Ohio, is an example of a commercially available product of this type. A bath temperature of about 170 F. has proved to be satisfactory. The article to be electroplated is clamped onto an electrode and immersed in the bath with the article as the anode. A 3-volt applied potential can be used and the current polarity reversed and adjusted to about 0.75 ampere to 1.5 amperes per square inch for a period of one-half to one hour. Next, the voltage is reduced to approximately 2 volts, and the article is removed and rotated to expose the previous contact points. The molybdenum article is then replaced in the bath at the 2-volt level, the voltage being increased to the plating level and the plating continued from one-half to one hour. This latter sequence of steps is repeated until the desired plating thickness is produced, after which the plated article is rinsed in water and dried. In general, we have found that four contact rotations are desirable if the molybdenum base metal article is to be completely coated.

The chromium plated article is then annealed for one hour at a temperature of 2000 F. in a substantially dry, flowing hydrogen atmosphere. In order to obtain the proper dryness of the hydrogen, tank hydrogen can be passed through a catalytic reactor to combine with any impurity oxygen present to form water. The gas is then dried by passing it through a cooling coil chilled by a solid carbon dioxide-acetone mixture. The dew point of this gas is approximately 90 to 100 F.

Following this annealing treatment for the chromium plate, the powdered mixture of 80% nickel and 20% chromium, to which has been added by weight of the aforementioned nicikel-chromiumboron powdered alloy, is flame sprayed to uniformly cover the electroplate. Ne have obtained excellent results with a powder of approximately l50 mesh. Our work indicates that 0.12 to 0.13 gram of nickel-chromium alloy overlay per square centimeter of the original surface area produces highly desirable results. After this flame spraying step, the as-sprayed nickel-chromium alloy coating is annealed in dry hydrogen for two hours at a temperature of about 1700 F. and for an additional hour at 2000 F. The coated article is then cooled to room temperature under the hydrogen atmosphere.

Next, a layer of the above-described powdered chromium-nickel-boron alloy is fiarne sprayed to uniformly cover the surface of the plated article. In general, we have found that the preferred weight range is approxi mately 0.12 to 0.13 gram per square centimeter of sprayed surface. The article is then fusion heat treated for about minutes at a temperature of approximately 2200 F. in a dry hydrogen atmosphere. It is desirable to charge the cold sample in the hot furnace while under this atmosphere. When the coated molybdenum article is still in the furnace, it is advantageous to subject it to a diffusion annealing treatment by lowering the furnace temperature to 2000 F. for one hour, raising it to 2l00 F. for one hour, followed by a one-hour anneal at 2200 F. Such a procedure paces the diffusion rate of boron into the matrix 80% nickel-% chromium alloy.

The advantages of the coating described herein are apparent when compared with the coatings heretofore used for the same purpose. For example, silicide coatings,

which are extremely resistant to oxidation at very high temperatures, are so brittle that they cannot withstand impact blows or any significant amount of stress applied to the underlying molybdenum. Aluminunrchromiumsilicon coatings likewise exhibit good oxidation resistance and have excellent thermal shock resistance, but such coatings fail to protect the molybdenum when impacted. The lack of ductility of these two types of coatings results from the fact that the primary oxidation resistance is provided by brittle inter-metallic compounds.

Various other coatings for molybdenum have been tried in the past. Such treatments include the simple application of nickel-chromium-boron alloy coatings, electrodeposited nickel and nickel-chromium alloy combinations and claddings. While coatings of this type are very ductile, they generally fail to protect molybdenum at temperatures above 2000 F. during extended exposure in air. Moreover, the application of such coatings recrystallizes the underlying molybdenum, :and the coatings have only fair thermal shock resistance. On the other hand, the protective coating which is produced by the procedure embodying the present invention can be applied without recrystallizing the base metal and without sacrificing ductility. Moreover, the thermal shock resistance is improved and the effective maximum exposure temperature is increased approximately 200 F.

While our invention has been described by means of certain specific examples, it is to be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. A process for forming a coated molybdenum base metal article which is resistant to oxidizing gases at temperatures above 2000 F., said process comprising coating surfaces of said article with a thin, crack-free layer of chromium, flame spraying a thin porous coating of an alloy comprising about 50% to nickel "and 15 to 50% chromium over said chromium layer, thereafter flame spraying a thin layer of an alloy comprising about 74% to 89.5% nickel, 10% to 20% chromium and 0.5% to 6% boron over said coating, and diffusing said nickelchromium-boron alloy into said nickel-chromium alloy.

2. A method of protecting a molybdenum base metal article against oxidation in air at elevated temperatures by providing said article with a ductile oxidation-resistant surface coating, said method comprising applying a thin layer of relatively soft, crack-free chromium on said article, heating said article to further soften said chromium layer and establish a diffusion bond to the molybdenum base metal, flame spraying a thin porous coating of nickel-chromium alloy comprising about 50% to 85% nickel and 15% to 50% chromium over said chromium layer, subsequently reducing any metal oxides generated and entrapped during said flame spraying, flame spraying a thin layer of nickel-chromium-boronalloy comprising about 74% to 89.5% nickel, 10% to 20% chromium and 0.5% to 6% boron over said porous coating, thereafter heating said article to melt said nickel-chromium boron alloy and cause it to impregnate said porous coating, and continuing to heat said article to reduce local boron concentration gradients in the coating by solid state diffusion into said nickel-chromium alloy.

3. A method of protecting a molybdenum base metal article against high temperature oxidation in air, said method comprising electroplating a thin layer of chromium on surfaces of said'article, heating said article in a noncarburizing, nonoxidizing atmosphere for a period of time and at a temperature sufficient to soften the chromium plate and establish a diffusion bond to the molybdenum base metal, flame spraying a thin porous coating of a metal comprising about 15% to 50% chromium and 50% to 85% nickel over said chromium, subsequently heating said article in said atmosphere at a temperature and for a period of time sufficient to reduce any metal oxides generated and entrapped during said flame spraying, flame spraying a thin powdered alloy layer comprising approximately to 20% chromium, 0.5% to 6% boron and the balance principally nickel over the formed nickel-chromium alloy coating, and heating said article in said atmosphere at a temperature and for a period of time sufficient to melt the nickel-chromium boron alloy and cause it to penetrate the pores of the nickel-chromium alloy;

4. A process for forming a protective coating on a molybdenum base metal article which comprises electroplating a thin barrier layer of chromium on surfaces of said article, heating said plated article in a substantially dry hydrogen atmosphere for a period of time and at a temperature sumcient to soften the chromium plate and establish a diffusion bond to the molybdenum base metal, thereafter flame spraying a powdered metal comprising about to 50% chromium and the balance substantially all nickel over said chromium layer in an amount equal to approximately 0.1 to 0.15 gram per square centimeter, subsequently heating said article in said atmosphere at a temperature and for a period of time sufficient to sinter the formed nickel-chromium alloy overlay and reduce any metal oxides generated and entrapped in said overlay during said flame spraying, flame spraying a powdered alloy comprising about 10% to chromium, 0.5% to 6% boron and the balance principally nickel over said overlay in an amount equal to about 0.1 to 0.15 gram per square centimeter, and thereafter heating said coated article in said atmosphere at a temperature and for a period of time sufficient to melt the nickelchromium-boron alloy and cause it to infiltrate pores of said sintered nickel-chromium alloy.

5. A process for forming a protective coating on an article formed of a base metal selected from the class consisting of molybdenum and molybdenum base alloys, said process comprising coating surfaces of said article with a thin, crack-free layer of chromium, applying a thin porous coating of a nickel-chromium alloy comprising about to 85% nickel and 15% to 50% chromium over said chromium layer, applying a thin layer of nickelchromiurn-boron alloy comprising about 74% to 89.5% nickel, 10% to 20% chromium and 0.5% to 6% boron over said coating, thereafter heating said article under nonoxidizing conditions to melt said nickel-chromiumboron alloy and cause it to penetrate pores of said nickelchrornium alloy, and continuing to heat said article to reduce local boron concentration gradients in the coating by diffusion into said nickel-chromium alloy.

6. A process for forming a protective coating on an article formed of a base metal selected from the class consisting of molybdenum and molybdenum base alloys, said process comprising applying to surfaces of said article a relatively soft, crack-free layer of chromium having a thickness of about 0.5 mil to 2 mils, annealing said chromium plated article at a temperature of approximately 2000 F. to 2200 Fjin a substantially dry nonocarburizing atmosphere which is reducing with respect to chromium, forming a porous layer of nickel-chromium alloy over said chromium layer in an amount equal to about 0.1 to 0.15 gram per square centimeter, sintering said porous layer at a temperature of about 1700 F. to.2200 F. in said atmos-' phere, thereafter applying a porous coating of an alloy comprising approximately 10% to 20% chromium, 0.5%

to 6% boron and the balance principally nickel over said nickel-chromium alloy layer in an amount equal to about 0.1 to 0.15 gram per square centimeter, and heat' treating' said article in said atmosphere at a temperature of ap- 7; A process for forming a ductile; high temperature oxidation-resistant coating on a molybdenum base metal proximately 1800 F. to 2250 Fato melt the nickelchromium-boron alloy coating and cause it to impregnate; said porous nickel-chromium alloy layer; and continuing to diffusion heat said article to reduce local boron concenarticle which comprises cleaning surfaces of a molybdenum base metal article, electroplating a relatively soft, crack-free layer of chromium on said article to a depth of about 0.5 mil to 4 mils, flame spraying a porous overlay of a metal powder on said chromium plate in an amount equal to approximately 0.1 to 0.15 gram per square centimeter, said powdered metal comprising about 15 to 50% chromium and the balance substantially all nickel, sintering said overlay in a substantially dry hydrogen atmosphere at a temperature of about 1700 F. to 3000 F., flame spraying a powdered alloy comprising approximately 10% to 20% chromium, 0.5% to 6% boron and the balance principally nickel on said overlay in an amount equal to about 0.1 to 0.15 gram per square centimeter, and thereafter heating said article in said atmosphere at a temperature of approximately 1800 F. to 2250 F. to melt the nickel-chromiumboron alloy and cause it to penetrate into the pores of said overlay, and continuing to heat said article to reduce local boron concentration gradients in the resultant coating by solid state diffusion.

8. A process for forming a ductile, high temperature oxidation-resistant coating on a molybdenum base metal article which comprises electroplating relatively soft, crackfree chromium on said article to a depth of about 0.5 mil to 2 mils, heating said plated article in a substantially dry, nonoxidizing, noncarburizing atmosphere to further soften the chromium plate and establish a diffusion bond to the molybdenum base metal, thereafter flame spraying over said chromium plate a thin porous layerof a metal comprising approximately 15% to 50% chromium and the balance substantially all nickel, sintering said layer in said atmosphere at a temperature of about 1700 F. to 3000 F., thereafter flame spraying a thin coating of powdered alloy comprising about 10% to 20% chromium, 0.5% to 6% boron and the balance principally nickel over said layer and heating said article in said atmosphere at a temperature of approximately 1800 F. to 2250 F. to

melt said nickel-chromium-boron alloy and cause it to penetrate into said porous layer, and continuing to diffusion heat said article in said atmosphere to reduce local boron concentration gradients in the resultant coating by diffusion into the nickel-chromium.

9. A process for forming a protective coating on a turbine blade formed of a base metal selected from the class consisting of molybdenum and molybdenum base alloys,

.said process comprising electroplating relatively soft,

crack-free chromium on said turbine blade to a depth of about 0.5 mil to 4 mils, heat treating said chromium plated turbine blade at a temperature of approximately 2000 F.

to 2200 F. in a substantially dry hydrogen atmosphere for a time sufficient to further soften the chromium plate and establish a diifusion bond to the base metal,thereafter flame spraying a porous overlay of a powdered metal on said chromium platein an amount equal to about 0.1 to 0.15 gram per square centimeter, said powdered metal comprising approximately 15% to 25% chromium and to nickel, heat treating said overlay in a substantially dry hydrogen atmosphere at a temperature of about 1700 F. to 2200 F. for a period of time suflicient to reduce any metal oxides generated and entrapped during said flame spraying, thereafter flame spraying a powdered alloy comprising approximately 10% to 17% chromium, 2% to 4% boron and the balance principally nickel on said overlayin an amount equal to about 0.1 to 0.15 gram per square centimeter, subsequently heat treating said plated turbine blade in a substantially dry hydrogen atmosphere at a temperature of approximately 2000 F. to 2250 :F. to melt the nickel-chromium-boron alloy and cause it to penetrate said porous overlay, and continuing to heat'said turbine blade insaid atmosphere to reduce localboron-concentration gradients in the resultant coating by solid state diffusion into the nickel-chromi- 10. A process for forming a protective coating ,on a molybdenum base metal article which comprises cleaning surfaces of a molybdenum base metal article, electroplating relatively soft, crack-free chromium on said article to a depth of about 1 to 2 mils, annealing said chromium plated article at a temperature of approximately 2000 F. to 2200 F. in a substantially dry hydrogen atmosphere for a time suflicient to further soften the chromium plate and establish a diftusion bond to the molybdenum base metal, thereafter flame spraying a porous layer of a metal powder having particle sizes of approximately 60 to +200 mesh over said chromium plate in an amount equal to about 0.12 to 0.13 gram per square centimeter, said metal powder comprising an alloy of approximately 15% to 25% chromium and the balance substantially all nickel to which has been added a nickel-chromium-boron alloy, subsequently sintering said layer in said atmosphere at a temperature of about 1700 F. to 2200 F. for a period of time suflicient to reduce any metal oxides generated and entrapped during said flame spraying, cooling said article to room temperature under said atmosphere, thereafter flame spraying a metal powder coating comprising approximately to 17% chromium, 2% to 4% boron and the balance principally nickel over said nickel-chromium alloy layer in an amount equal to about 0.12 to 0.13 gram per square centimeter, said powder having particle sizes of approximately 60 to +200 mesh, and fusion heat treating said article in said atmosphere at a temperature of approximately 2100 F. to 2250 F. to melt the nickel-chromium-boron coating and cause it to infiltrate the pores of said porous layer, and difiusion heating said article in said atmosphere at a temperature of about 2000 F. to 2250 F. to reduce local boron concentration gradicuts in the resultant coating by solid state diflusion into the nickel-chromium.

11. A molybdenum base metal article having a surface coating characterized by high temperature oxidation re sistance and excellent ductility with respect to impact indentation and creep deformation, said article comprising a molybdenum base metal, a barrier layer of relatively soft,

crack-free chromium on surfaces of said base metal and bonded thereto, said chromium layer having a thickness of approximately 0.5 mil to 4 mils, and a thin coating comprising a matrix of nickel-chromium alloy over said barrier layer and fused thereto, said matrix being impregnated with an alloy comprising about 10% to 20% chromium, 0.5% to 6% boron and the balance substantially all nickel, the local boron concentration gradients in said coating having been reduced by diffusion into the nickel-chromium alloy.

12. A turbine blade having a surface coating characterized by high temperature oxidation resistance and excellent ductility with respect to impact indentation and creep deformation, said turbine blade comprising a base metal selected from the class consisting of molybdenum and molybdenum base alloys, a diffusion barrier layer of relatively soft, crack-free chromium electroplated on surfaces of said base metal and diffusion bonded thereto, said barrier layer having a thickness of approximately 0.5 mil to 2 mils, and a thin coating comprising a matrix of a nickelchromium alloy over said barrier layer and fused thereto, said matrix being present in an amount equal to about 0.1 to 0.15 gram per square centimeter and comprising about 15% to 25% chromium and the balance substantially all nickel, said matrix being infiltrated with an alloy comprising approximately 10% to 17% chromium, 2% to 4% boron and the balance substantially all nickel, the local boron concentration gradients in said coating having been reduced by solid state diffusion into the nickel-chromium alloy, the coating on said turbine blade having a total thickness of about 5 to 20 mils.

Turner et al Sept. 25, 1956 Montgomery et al Dec. 25, 1956

Claims

7. A PROCESS FOR FORMING A DUCTILE, HIGH TEMPERATURE OXIDATION-RESISTANT COATING ON A MOLYBDENUM BASE METAL ARTICLE WHICH COMPRISES CLEANING SURFACE OF A MOLYBDENUM BASE METAL ARTICLE, ELECTROPLATING A RELATIVELY SOFT, CRACK-FREE LAYER OF CHROMIUM ON SAID ARTICLE TO A DEPT OF ABOUT 0.5 MIL TO 4 MILS, FLAME SPRAYING A POROUS OVERLAY OF A METAL POWDER ON SAID CHROMIUM PLATE IN AN AMOUNT EQUAL TO APPROXIMATELY 0.1 TO 0.15 GRAM PER SQUARE CENTIMETER, SAID POWDERED METAL COMPRISING ABOUT 15% TO 50% CHROMIUM AND THE BALANCE SUBSTANTIALLY ALL NICKEL, SINTERING SAID OVERLAY IN A SUBSTANTIALLY DRY HYDROGEN ATMOSPHERE AT A TEMPERATURE OF ABOUT 1700*F. TO 3000*F., FLAME SPRAYING A POWDERED ALLOY COMPRISING APPROXIMATELY 10% TO 20% CHROMIUM, 0.5% TO 6% BORON AND THE BALANCE PRINCIPALLY NICKEL ON SAID OVERLAY IN AN AMOUNT EQUAL TO ABOUT 0.1 TO 0.15 GRAM PER SQUARE CENTIMETER, AND THEREAFTER HEATING SAID ARTICLE IN SAID ATMOSPHERE AT A TEMPERATURE OF APPROXIMATELY 1800* F. TO 2250* F. TO MELT THE NICKEL-CHROMIUM-BORON ALLOY AND CAUSE IT TO PENETRATE INTO THE PORES OF SAID OVERLAY, AND CONTINUING TO HEAT SAID ARTICLE TO REDUCE LOCAL BORON CONCENTRATION GRADIENTS IN THE RESULTANT COATING BY SOLID STATE DIFFUSION.