US2690409A - Binary coating of refractory metals - Google Patents

Description

Sept. 28, 1954 E WAlNER 2,690,409

BINARY COATING F REFRACTORY METALS Filed July 8, 1949 Patented Sept. 28, 1954 BINARY COATING OF REFRACTORY METALS Eugene Waner, Cleveland, Ohio, assignor to Thompson Products, Inc., Cleveland, Ohio, a

corporation of Ohio Application July 8, 1949, Serial No. 103,632

(Cl. 14S-31.5)

4 Claims.

The present invention relates to a method of coating refractory metal articles to enhance their resistance to high temperature and corrosive atmospheres. The present invention specifically relates to the manufacture of coated refractory metal turbine buckets for use in jet turbines and the like.

Turbo-jet engines or the like are usually provided with an axial ow turbine operated by exhaust gases which drive a blower furnishing air to the burners. Such turbines operate at extremely high temperatures, and one of the major difiiculties encountered in the manufacture of jet turbines has been the provision of suitable material for bucket blades which can withstand the effect of such high temperatures. The turbine bucket will normally be exposed to temperatures in the range of from 1600 to 2000 F. and the bucket must have sufiicient strength, toughness, creep resistance, and resistance to the corrosive atmosphere present to enable the bucket to operate efficiently without deformation or corrosion.

In addition to turbine buckets, articles produced by the present invention may be employed under conditions of higher temperature and lower stress than exist in a gas turbine bucket. One such application occurs in nozzle diaphragm vanes in gas turbines which must withstand very severe conditions of temperature and thermal shock 'but at a relatively lower stress.

One refractory metal which exhibits excellent properties of strength, toughness and creep resistance at elevated temperatures is molybdenum. However, metallic molybdenum itself cannot be used. The trioxide of molybdenum, which is formed under the oxidizing conditions present in the turbine, sublimes at a temperature of about 1463 F. at an extremely rapid rate. This phenomenon gives rise to a characteristic smoking when bodies of molybdenum are heated to tempera-tures above 1463" F., resulting in the complete disappearance of the molybdenum within a matter of minutes. Another refractory metal which might be used for turbine bucket bodies is tungsten, even though it has a relatively high density.

To overcome this difficulty, I have herein provided a process for coating refractory metal such as molybdenum to provide thereon a tough, corrosion-resistant coating impervious to the eect of oxygen and other gases.

It is then an object of the present invention to provide a method for coating refractory metals such as molybdenum to provide an extremely corrosion resistant surface thereon.

Another object of the present invention is to provide a method for coating refractory metals which yields a rm bond to the refractory surface and makes it impervious to the effects of operation under conditions of high stress and high temperatures.

A further object of the invention is to provide a coated molybdenum article, such as a turbine bucket, capable of operation within a turbine engine for extended periods of time without deteroration.

In the process of the present invention, the refractory metal is coated with a metal selected from the group consisting of silicon, aluminum, and zirconium and the coating is subsequentlyreacted with another element to produce a binary coating. The coating and reaction steps may be carried out concurrently, or the refractory metal may be given a primary coat of silicon, aluminum or zirconium and subsequently reacted with the second element, which is preferably selected from the group consisting of the elements in groups III, IV or V of the periodic table.

The element which is to be reacted with the primary coating is one whose ionic size is close enough to the ionic size of the primary metal coating to allow the elements to be mutually soluble with each other in the solid state, and thus increase the rate of intermetallic compound formation. Such intermetallic compounds per se or the compounds formed with the base metal then have a size which approximates the atomic spacing in the lattice of the base metal. The primary coating inherently leaves microscopic voids, tunnels or weak spots in the surface of the article, thus decreasing its ability to withstand corrosion. By reacting the primary coating with a second element of the type menticned, it is believed that the voids in the atomic spacing of the single metal are iilled by the reaction with the second element, thus making the layer much less permeable. The reaction with the second element depends primarily upon the spacing in the crystal lattice of the primary coating. Where the primary coating is silicon, the secondary element may be titanium, zirconium, boron, aluminum, nitrogen, or carbon. In the case of aluminum, a primary coat of this metal may be further reacted with zirconium, titanium, chromium, boron, tin, and nickel. Where zirconium is used as the primary coat, the subsequent reaction may be carried out with aluminum, boron, carbon, silicon, titanium, and nitrogen.

The compounds resulting from the reaction of the second named elements with the primary coating are complex intermetallic compounds Which exhibit the property of forming an eX- tremely firm bond to the surface of the refractory metal. It is believed that this rm bond results from the closure of voids in the atomic lattice of 'the-base metal, but the present invention is in no way limited to the correctness of the given theory.

The coating process of the present invention may be most conveniently carried out in a vapor phase deposition system of the type described in a copending application Serial No. 98,272, led June 10, 1949 by myself and Robert A. Kempe. In that application, there is described a Amethod for coating refractory metals wherein a decomposable compound, preferably a halide, of the coating metal is carried into a reaction zone in a stream of hydrogen and therein decomposedto form a layer of substantially pure metal on the surfaces of the refractory metal. Deposition of thecoating metal compound, as discussed in this previous application, vis-the result of several factors. Some of the compound is probably decomposed vby the high temperatures,on the order of 1600 to 2300 F. present in the coating Zone. Another portion of 'the decomposable coating compound is reducedby the presence of the hydrogen atmosphere in which the compound is introduced. Another reaction which -occurs is the metathetical reaction between the coating metal compound and the -molybdenum wherein the coating metal is deposited on the molybdenum with `the rformation of a volatile molybdenum compound in the-exchange reaction.

The decom'posable compoundlemployed in the primary coating step is preferably a halide, for example silicon tetrachloride, trichlor silane, silicon-tetrabromide, tribromo silane, silicon tetraiodide, aluminum chloride, valuminum bromide, aluminum iodide, vzirconium chloride, zirconium bromide, or zirconium iodide. vWhere the coating metal compound exists `normally in the liquid state, vas is in the case-of silicon tetrachloride, the coating metal compound may be introduced into the 'heated reaction AZone Vby merely passing a stream of hydrogen gas through a liquid pool of the compound so -that vapors thereof are carried by the hydrogen into the reaction zone. Where the coating metal compound `is normally solid, the hydrogen `gas vmay be passed `over a heated source ofthe c'ompound'in powder vform 'to carry it' into'th'e reaction zone.

lIn reacting the primary coating with the'second element, a decomposable compound capable of yielding the second element is introduced into thereac'tion zone either prior to, during, or after the deposition of the primary coating. vTypical decomposable compounds which may be used in this connection are titanium tetrachloride, titanium tetrabromide, chromium chloride, nickel chloride, boron chloride, and tinchloride. Where thepri'mary metallic 'coating is to be carburized, i. e., reacted with carbon, a carburiz'ing gas is introduced into the reaction zone. Any common carburizing gas usuch as -natural gas, lcarbon monoxide, ethane, propane, `lnitane, and benzene maybe used, or the carburizing operation may be carried out in carburizing packs, or in liquid baths.

Where the primary coating is to .be nitrided, i. e., reacted with nitrogen, ammonia gas Vis preferably introduced into the reaction chamber to react .with'the primaryscoating. In addition to 4 ammonia gas, various decomposable cyanides may be employed.

The reaction conditions in the coating Zone include temperatures from 1600u to 2300 F. and preferably from 1700 to 2l00 F. and reaction times ranging from 4 to 24 hours depending upon the vpenetration required. Normally, the depth of the primary coating will -be on the'order of about 0.0003 to about 0.003 inch.

AWhere silicon is used as the primary coating metal, vthe ,outermost layer is substantially pure silicon, the nextlayer is probably a molybdenumsiliccn compound having a relatively high silicon content, possibly MoSiZ while the innermost layer producedby "the coating process is essentially a molybdenum-'silicon compound having a high molybdenum concentration, such as MozSi together with solid solutions of molybdenum and silicon. rhis type of structure is one whichis stable at high temperatures, and provides an excellent intimate bond with the .-molybdenum base. The reaction with 'the Ysecond element of the type mentioned above enhances'the properties of the coating by filling up the microscopic voids and Weak spots, resultingin a substantial increase in stability andresistance to high temperature oxidation.

VA further description of the present invention will .be made in connection with the attached sheet of drawings in which:

Figure 1 is aow sheet showing in'generalthe various stages of the coating process; and

Figure v2 is a drawing of a iphotomicrograph taken as a magnication of 500K showing the crystal structure of a molybdenum article coated in .accordance with the `present invention.

As'shown on the drawings:

Reference numeralV I0 denotes a'supply of purging gas, which is preferably an inert gas such as nitrogen, argon, neon,'he1ium or the like which is passed .into apurication zone I I where moisture and Vother contaminantsare removed. The purication zone II may consist of a supply `of liquid sulfuric `acid through whichthe purging gas 'is bubbled. v'The `purified gas is next introduced into a heated 'furnace IZWhichsurrounds a Vfurnace tube I3, control of .the gas owing into the furnace'tube being .controlled by `means of a valve I4.

Disposed Vwithin the Vfurnace tube I3 are a plurality of boats L5 Ywhich carry 'a number of turbine buckets I6 or other article's composed of a refractory metal such as .-.molybdenum- These articles are `normally preshaped into their 'desired form, and Worked at temperatures below the recrystallization temperature Aof the metal, to enhance its physical properties. The temperature of the furnace I2 is regulated between 1600 and 2?00o F., with .1700to 2100o F. being a preferred range.

A supply of hydrogen gas I1 is provided for introduction into thefurnacetube I3. VPrior to its introduction into the furnace tube I3, the hydrogen gas is dehydrated andpurined by means of various desiccants such `asjfor example, packed columns of silica gel, calcium chloride, 'or liquid sulfuric acid in a puriiication stage I8. The owofthe hydrogen gas into the furnacetube I3 .is vcontrolled by `'means Vof a valve vI9. .A secondfsource of hydrogen gas 20 and its purication stage 2| is also providedto act asa lcarryingmedium'for the coating compounds. `A source of the primary coating-compound A, normally a decomposablehalide, kis maintained in Va zone 22. A source of a compound, -B, of .the Ysecond `reactive element which may be one of the metals given previously, or a carburizing or nitriding gas is indicated at zone 23. Flow into the zones 22 and 23 is regulated by means of the respective valves 2li and 25 while flow of hydrogen gas containing the coating compounds is regulated by means of the valves 23 and 21 at the exit of stages 22 and 23. In this arrangement, either of the coating compounds may be introduced separately into the reaction zone or the introduction `of both compounds may be made simultaneously.

Initially, the furnace tube I3 is purged by means of the purging gas I9 to rid the furnace chamber of moisture and oxygen or other undesirable contaminants. Thereupon, the hydrogen and the coating metal compound which it carries is introduced into the furnace tube i3, Where it contacts the molybdenum article I6 for periods of time ranging from 4 to 24 hours. Excess hydrogen is vented from the furnace tube by means of tube 23. The molybdenum articles I6 are maintained in the furnace tubes I3 until a coating having a thickness of approximately .0093 to .093 inch is produced on the article.

In Figure 2, there is shown a drawing of a photomicrograph oi a molybdenum turbine bucket produced by simultaneous vapor deposition of silicon and titanium for a period of eight hours at a temperature of 2000 F. As illustrated in that drawing, the body of the article consists of rather large crystals of molybdenum 29 having an overlying layer of silicon 30. Immediately above the silicon layer 30 is a layer 3l of indeterminate composition, which is probably a complex mixture of molybdenum, titanium and silicon in the form of various intermetallic compounds. The uppermost layer 32 is a mixture of titanium and silicon compounds of these two elements. This structure has been found capable of withstanding operation in a gas turbine operating at temperatures estimated at 1600" to 1800 F. for periods in excess of 100 hours without apparent deterioration.

From the foregoing, it will be appreciated that I have herein provided a process for coating refractory metals which are in themselves incapable of withstanding the corrosive effect encountered in the operation of a gas turbine.

The coatings produced in accordance with this invention are integrally bonded to the base metal and cannot be stripped mechanically from the body metal, as is the case of coatings applied by electroplating or dipping. Further, the coatings are inert with respect to the molybdenum base metal and show no evidences of reaction with the base metal after the original deposition.

The vapor phase deposition processes described herein have complete throwing power i. e., a uniform coating can be deposited over the entire surface of the article regardless of corners, grooves or other irregularities in the surface of the article. This is not true of other types of coating procedures.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. The method of providing a corrosion resistant surface on a molybdenum article which comprises depositing a layer of silicon on said article, and nitriding the silicon layer.

2. A molybdenum body having a corrosion resistant outer layer of nitrided silicon thereon.

3. The method of providing a molybdenum body with a corrosion resistant coating, which comprises depositing on the surface of said body a layer consisting essentially of an element selected from the group consisting of silicon and zirconium, and reacting the selected element in said layer with a different element selected from the group consisting of silicon, zirconium, titanium, boron, aluminum and nitrogen to form a binary coating on said body, said coating being integrally bonded to the molybdenum` body.

4. A molybdenum body having an outer corrosion resistant coating thereon consisting essentially of the reaction product of an element selected from the group consisting of silicon and zirconium with a different element selected from the group consisting of silicon, zirconium, titanium, boron, aluminum and nitrogen, said coating being integrally bonded to the molybdenum body.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,551,333 Schroter et al. Aug. 25, 1925 1,929,252 Morris -1 Oct. 3, 1933 2,096,924 Schwartzkopf Oct. 26, 1937 2,235,504 Rennie Mar. 18, 1941 2,294,562 Kingston Sept. 1, 1942 FOREIGN PATENTS Number Country Date 842,981 France June 22, 1939 OTHER REFERENCES Metals Handbook, 1939 edition, pages 1054, 1055, 1074, 1090. Published in 1939 by the American Society for Metals.

Claims (1)

1. THE METHOD OF PROVIDING A CORROSION RESISTANT SURFACE ON A MOLYBDENUM ARTICLE WHICH COMPRISES DEPOSITING A LAYER OF SILICON ON SAID ARTICLE, AND NITRIDING THE SILICON LAYER.

Images