Vapor Phase Deposition Of Silicide Refractory Coatings

Abstract

Refractory coatings are formed upon the periphery of objects by passing a vaporous reactant stream in contact with the objects which are tumbled in a rotary deposition zone. In addition, reactant vapors are passed through a reactant generating zone positioned adjacent and preferably enclosed within the rotary deposition zone which is heated to the reaction temperature to thereby generate at least a portion of the reactant vapors utilized in the deposition of the refractory coating. For example, titanium tetrachloride vapors are passed over chromium and titanium metal chips positioned within the generation zone to produce chromium dichloride and titanium trichloride reactant vapors. A suitable apparatus is also provided for generating at least a portion of the vaporous reactants and combining them with other vaporous reactants and directing them onto tumbling objects to be coated within a rotary deposition chamber.

Description

This invention relates to vapor phase deposition of refractory materials upon a substrate. In another aspect, this invention relates to an improved method and apparatus for forming a continuous refractory coating on the periphery of an object.

Refractory metals, such as tantalum, molybdenum, columbium (niobium) and tungsten, for example, are especially desirable for applications requiring high strength at elevated temperature, particularly where ease of fabrication and ductility are needed. However, before any of these metals can satisfy a wide range of requirements, they must receive a suitable protective coating, such as an oxidation resistant and/or an abrasive resistant coating. For example, titanium and chromium, being metallurgically compatible with the refractory metals, form a good base coating which, when alloyed with silicon, or reacted with carbon or nitrogen will protect them against oxidation, and/or abrasion.

Various chemical vapor phase deposition processes have been developed for depositing protective coatings on substrates, including substrates of refractory metals. These protective coatings include silicon carbide, titanium carbide, silicon nitride, titanium nitride, metal carbonitrides, and titanium-chromium-silicon alloys, to name a few. When depositing some of the newer solid solution or alloy layers, problems have been encountered in the formation and maintaining of suitable vaporous reactant streams. For example, attempts to co-reduce titanium and chromium halides produced separately, and individually introduced into a deposition chamber, failed to yield satisfactory co-deposits of titanium and chromium alloys upon the substrate. This is true because of resultant non-uniform mixing of the two halide gas streams and inadequate control of the composition of the reactant gas stream. Furthermore, in the separate production of the halides of the metals by the reaction of hydrogen chloride gas, for example, with the individual metals, the unreacted hydrogen chloride gas tends to inhibit the deposition of the titanium metal. Thus, the formation and control of the vaporous reactant streams has been conventionally very difficult to accomplish.

Additionally, problems have been encountered in obtaining uniform deposits of a protective coating about the entire periphery of an object, particularly irregularly shaped objects such as nuts and bolts, which are used for high temperature fasteners, such as for example, fasteners for a rocket nose cone. It is essential that the entire periphery of these objects obtain a uniform coating to prevent damaging corrosion from eroding and weakening any portion thereof. Therefore, it is necessary that the entire periphery of these objects be subjected to the vaporous reactant atmosphere.

One object of this invention is to provide an improved process and apparatus for forming a protective coating on the periphery of an object.

Another object of this invention is to provide an improved process and apparatus for forming and effectively mixing a vaporous reactant stream and directing it upon a tumbling object to be coated in a rotating vapor deposition zone.

A further object of this invention is to provide a method and apparatus for applying a uniform refractory or protective coating on the periphery of an object by vapor phase deposition, and for generating at least a portion of the vaporous reactant stream.

According to one embodiment of this invention, a process is provided for applying a protective coating to the periphery of an object comprising placing the object within a rotary deposition zone which causes the object to tumble therein, and directing a vaporous reactant stream in contact with the tumbling object wherein at least a portion of said stream is generated by passing a reactant halogen-containing stream over metal particles in a generation zone to form a corresponding metal halide. Preferably, the generation zone is located within a heat zone containing the rotating vapor deposition zone.

According to another embodiment of this invention, an apparatus is provided for carrying out the above-described process which generally includes a rotary vapor phase deposition chamber for receiving and imparting a tumbling motion to at least one object to be coated with vapor phase reactant product, an oven means for enclosing the deposition chamber, nozzle means for introducing vapor phase reactants into the rotary vapor phase deposition chamber, and at least one reactant generation chamber positioned within said oven means and communicating with said nozzle means .

This invention can be more easily understood from a study of the drawings in which:

FIG. 1 is an elevational view of a vapor phase deposition apparatus of this invention with the rotary deposition chamber positioned outside of the oven;

FIG. 2 is the apparatus of FIG. 1, partially in section, showing the rotary vapor phase deposition chamber extended within the oven; and

FIG. 3 is a partial sectional view of FIG. 2 showing a nozzle means and reactant generation chamber thereof.

Now referring to FIG. 1, the apparatus of this invention is illustrated which generally comprises rotary vapor phase deposition reactor means 10 and an oven means 11.

Rotary vapor phase deposition reactor means 10 comprises rotary deposition chamber 12 rotatably mounted on tubular spindle 13, which is carried by frame 14. Frame 14 is slidably mounted on guide rods 15 and is actuated by worm gear 16 which passes through a cooperating threaded portion of member 17, which extends from frame 14. Crank 18 is operatively attached to the end of worm gear 16 through gear box 19. Thus, a rotational motion of crank 18 will cause frame 14 to move along guide rods 15 by the action of worm gear 16 on the threaded portion of member 17.

Now referring to both FIGS. 1 and 2, rotary deposition chamber 12 is mounted to flange 13a of tubular spindle 13 by suitable fastening means such as nut and bolt assemblies 20. Tubular spindle 13 is rotatably mounted between supports 21 and 22 and has nozzle means 23 operatively positioned therethrough in a non-rotating, fixed relationship. Sprocket 24 is affixed to the periphery of spindle 13 and is attached by a drive chain 25 to driving sprocket 26 of motor 27.

Rotary deposition chamber 12 generally comprises a widened body portion 12a for receiving at least one object, such as nuts and bolts as illustrated in FIG. 2, and an elongated neck section 12b slightly wider in diameter than the width of nozzle means 23, and flange 12c, having apertures which match with apertures through flange 13a of spindle 13 for receiving nut and bolt assemblies 20. Rotary deposition chamber 12 can be made of any suitable ceramic material such as quartz.

The outlet of nozzle means 23 is positioned to direct reactants on the objects 50 positioned within the widened portion 12a of rotary deposition chamber 12. Reactant inlet conduits 28, 29 and 30 are operatively connected to nozzle means 23, as will be described in relation to FIG. 3, below. Outlet conduit means 31 operatively communicates with annular zone 32 between reactant nozzle means 23 and rotary deposition chamber 12.

Oven 11 carries a suitable heating chamber 33 (shown in broken line), which communicates with a pair of cooperating hinged oven doors 34. Each oven door carries a semi-circular recessed portion 35 on its outer edge for receiving elongated neck portion 12b of rotary deposition chamber 12 when in closed position.

Now referring to FIGS. 1-3, and particularly FIG. 3, nozzle means 23 is illustrated in detail. As shown in FIG. 3, nozzle means 23 generally comprises a tubular member 36 which is operatively connected to feed inlet conduit 30 within support member 22 and has a downturned outlet end 37. In addition, nozzle 23 carries generation chamber 38 therewithin. Conduits 39 and 40 communicate with reactant inlet conduits 29 and 28, respectively. The outlet of conduit 39 extends through tubular body 36 and is substantially concentric with downturned outlet 37. The outlet of tube 40 is positioned adjacent outlet 37. It is noted that any number of conduits 40 can be utilized within the scope of this invention according to the particular reactants utilized. For example, one or more conduits 40 can be positioned on nozzle means 23. If desired, the outlet end of conduit 40 can be positioned in a downturned manner adjacent outlet 37.

Reactant generation chamber 38 can comprise one or more chambers contained within porous plugs 41 and 42. As illustrated, generation chamber 38 carries a partition member 43, which separates the chamber into two individual generation chambers 38a and 38b containing either similar or dissimilar metallic particles 44 and 45, respectively. Holes 46 through porous plugs 41 and 42 can be varied in size and number to control the relative proportions of reactant gas flow through the two chambers 38a and 38b.

The process and apparatus of this invention finds particular utility in applying protective coatings to the periphery of objects with vaporous metal compounds which have relatively low vapor pressures at room temperature. In addition, this invention is especially useful for generating highly reactive metal sub-halide reactants from the more stable metal halides which have high vapor pressures at room temperature. The metal sub-halides are not only more reactive but result in a reduced amount of by-product acid vapor which generally inhibits the deposition rate on the substrate.

Thus, this invention can be utilized to apply various protective coatings to the surface of objects. However, it will be described in detail hereinbelow with reference to the deposition of a complex silicide refractory compound coating consisting of titanium, chromium, and silicon. The introduction of chromium to a plating environment is generally very difficult, since the vapor pressure of chromium is relatively low even at elevated temperatures. This invention will assure that chromium is introduced to the plating environment used in coating the periphery of an object. In addition, this invention can be used to form a more reactive halide specie of a vaporous metal halide reactant.

Thus, in a preferred embodiment, particulate chromium is positioned in one of the generating chambers, for example, 38b within tubular body 36, and particulate titanium is placed within the other generating chamber 38a. Rotary deposition chamber 12 is removed from spindle 13 and objects 50 are positioned therewithin. Next, rotary deposition chamber 12 is affixed to flange 13a of spindle 13 by nut and bolt assemblies 20. Hand crank 18 is actuated to move frame 14 along guide rods 15 toward oven 11. As rotary deposition chamber 12 extends within oven 11, oven doors 34 are closed in a manner as illustrated in FIG. 2 such that elongated neck portion 12b of rotary deposition chamber 12 extends between cooperating semicircular recesses 35.

The heating element of oven 11 is actuated to heat rotary deposition chamber 12, generation chamber 38, and objects 50 to a suitable reaction temperature, e.g., 900.degree. C. Next, motor 27 is actuated to cause drive chain 25 to rotate spindle 13 at a suitable speed, for example, from about 0.5 to about 15 rpm. This rotating action causes the rotation of rotary deposition chamber 12 and thereby causes the tumbling of parts 50 therewithin. Next, suitable reactants are supplied by conduits 28-30. Titanium tetrachloride is passed into generation chamber 38 from feed conduit 30. Silicon tetrachloride is passed into conduit 39 of nozzle means 23 from feed conduit 29, and reducing hydrogen is passed through conduit 40 of nozzle means 23 from feed conduit 28. Exhaust gases are removed from annular space 32 within rotary deposition chamber 12 by exhaust conduit 31. The action of the titanium tetrachloride on the chromium particles contained within chamber 38b is generally as follows:

2 TiCl.sub.4 + Cr .fwdarw.2 TiCl.sub.3 + CrCl.sub.2.

In addition, the reaction of the titanium tetrachloride on the titanium metal particles contained within generation chamber 38a is as follows:

3 TiCl.sub.4 + Ti .fwdarw.4 TiCl.sub.3.

Thus, a reactant feed stream consisting essentially of titanium trichloride and chromium dichloride is passed to outlet 37 of nozzle means 23 from porous plug 42. In addition, the vaporous reactants from porous plug 42 are intermixed with silicon tetrachloride passed from the outlet of conduit 39. The resulting reactant feed mixture is passed downwardly into the interior of rotary deposition chamber 12 in contact with the tumbling parts 50.

Hydrogen supplied to the interior of rotary deposition chamber 12 reduces the metallic halides emitted from outlet 37 of nozzle 23 when contact is made with an object 50 to form a complex silicide refractory compound consisting essentially of titanium, chromium, and silicon, thereby leaving a by-product of hydrogen chloride which is removed from deposition chamber 12 via conduit 31 as described above.

After the deposition process, reactant flow is cut off, oven doors 34 are opened, and hand crank 18 is actuated to thereby pull frame 14 from the interior of oven 11. The coated objects are then removed from rotary deposition chamber 12 after it is removed from spindle 13.

It must be understood that various vapor phase chemical deposition processes can be carried out according to this invention. For example, metallic particles utilized in generation chamber 38 can be metals selected from Groups IVB, VB, VIB, VIIB and VIII of the Periodic Table, carbon, silicon, and boron, or mixtures thereof. The reactant gas passed through generation chamber 38 can be any suitable reactant known in the art which will combine with the metal, volatilize the same, and later react with a suitable reducing agent such as hydrogen within the rotary deposition chamber. For example, metal halides, molecular halogen, and acid vapors such as hydrogen chloride.

Thus, this invention can be utilized to deposit uniform refractory coating such as metal carbides, metal nitrides, metal carbonitrides and silicide coatings on the periphery of objects tumbling within the rotary vapor phase deposition zone. For example, if it is desired to deposit a metal carbonitride, such as titanium carbonitride, on the surface of an object within the rotary deposition zone, titanium particles can be positioned within generation chamber 38, and titanium tetrachloride passed therethrough to form a feedstream of titanium trichloride. The oven in this instance can be heated to a temperature in the range from abour 400.degree. to about 1,200.degree. C. Next, molecular nitrogen and/or an easily decomposable nitrogen-containing compound, an easily decomposable carbon-containing compound (alternatively, an easily decomposable nitrogen and carbon-containing compound) is passed through conduit 39. Molecular hydrogen is passed through conduit 40.

Suitable carbon-containing reactant compounds include cyclic and acyclic hydrocarbons having up to about 18 carbon atoms which readily decompose at the deposition temperature. Examples of suitable hydrocarbons include the parafins, such as methane, ethane, propane, butane, pentane, decane, pentadecane, octadecane, and aromatics such as benzene and halogen substitute derivatives thereof.

Suitable reactant compounds containing both carbon and nitrogen include aminoalkenes, pyridines, hydrazines, and alkylamines. Some specific examples include diaminethylene, triaminoethylene, pyridine, trimethylamine, triethylamine, hydrazine, methylhydrazine, and the like.

Thus, the above described reactants can be utilized to form a titanium carbonitride coating on the surface of objects 50 tumbling within rotating deposition chamber 12.

This invention can be more easily understood from a study of the following examples which are given for illustrative purposes only.

EXAMPLE 1

In this example an apparatus was utilized which was substantially the same as that illustrated in the drawings. Widened portion 12a of rotary deposition chamber 12 comprised a cylindrically shaped body 5 inches in diameter and 6 inches long. Prior to the run, 10 1/4 inches diameter by 11/2 inches long hexhead bolts made of a refractory columbium alloy were placed within chamber 12, pure titanium metal chips were positioned within generating chamber 38b and pure chromium chips were placed within generating chamber 38a.

Next, frame 14 was moved forward and oven doors 34 closed so that widened portion 12a of rotary deposition chamber 12 was positioned within oven 11 as illustrated in FIG. 2. The interior of oven 11 was maintained at 900.degree. C. Motor 27 was actuated to cause spindle 13 and rotary deposition chamber 12 to rotate at a speed of 11 rpm.

Reactants were next passed in through reactant inlet conduits 28, 29 and 30. A stream consisting of 0.238 liter per minute of titanium tetrachloride and 17.3 liters per minute of argon was passed through inlet conduit 30 to tubular body 36 and through generation chamber 38. A stream of 0.12 liter per minute of silicon tetrachloride in 1.38 liters per minute of argon was passed through inlet conduit 29 and conduit 39 of nozzle means 23. Ten liters per minute of hydrogen was passed in through inlet conduit 28 and conduit 40 of nozzle means 23. This reactant flow continued for a period of 270 minutes. At the end of the deposition time, there was shown to be substantial weight loss of the chromium and titanium chips from generating chamber 38. Each bolt was found to have an average of 330.7 milligrams of silicide coating (a coating consisting essentially of titanium, chromium, and silicon) thereon. This coating was substantially uniform over the entire portion of the bolt including the threaded portion and resulted in a substantially oxidation resistant surface thereof. The coating thickness was between 2 and 3 mils, and oxidation tests at 2,400.degree. F in an oxidation environment showed an increased life of 80 times of that of similar uncoated bolts.

EXAMPLE 2

The apparatus as described above was utilized except in this instance, 10 1/4 inches diameter by 11/2 inches long hexhead bolts, 10 hex nuts for the bolts, and 10 1/4 inches diameter by 1 inch long threaded studs all made of a refractory columbium alloy were utilized as the substrates within rotary deposition chamber 12.

After the 30 substrates were loaded into the rotary deposition chamber 12, the run was conducted in substantially the same manner as described in relation to FIG. 1. However, furnace 11 was maintained at 990.degree. C and the flow of reactants was as follows: 0.140 liter per minute of titanium chloride and 10.2 liters per minute of argon were passed through generating chamber 38 containing the titanium and chromium chips; 0.084 liter per minute of silicon tetrachloride and 2.66 liters per minute of argon were passed through conduit 39 of nozzle means 23; and 7 liters per minute of hydrogen together with 3 liters per minute of argon was passed through conduit 40 to the interior of rotary deposition chamber 12. The flow of reactants continued for a period of 480 minutes.

After the run was completed, it was found that there was a substantial loss of weight of the chromium and titanium chips from generating chambers 38a and 38b. In addition, it was found that the average weight gain of the hexhead bolts was 528.2 milligrams per bolt, the average weight gain of each hex nut was 118.6 milligrams, and the average weight gain of each of the threaded studs was 147.6 milligrams. It was found that the silicide coating was uniform about the periphery of the objects, even on the threaded surfaces.

EXAMPLE 3

Example 2 was repeated, except 0.084 liter per minute of diaminethylene in 2.616 liters per minute of argon was passed through conduit 39 of nozzle 23 instead of silicon tetrachloride.

This procedure formed a uniform deposit of titanium carbonitride on the periphery of the individual substrates.

While this invention has been described in relation to its preferred embodiments, it is to be understood that various modifications of this invention will now become apparent to one skilled in the art upon reading the specification, and it is intended to cover such modifications as fall within the scope of the appended claims.

Claims

I claim:

1. A method of forming a protective coating on the surface of a compatible substrate which comprises:

contacting a particulate mass of elemental chromium and a particulate mass of elemental titanium with a reactant stream comprising titanium tetrachloride, thereby forming a reactant stream comprising titanium trichloride and chromium dichloride;

tumbling said substrate in a rotary deposition zone;

heating said particulate mass of chromium, said particulate mass of titanium, and said substrate in a common heating zone; and

contacting said substrate with silicon tetrachloride, hydrogen, and the reactant stream produced by reacting said titanium tetrachloride with the elemental titanium and chromium, thereby forming a refractory silicide coating on said substrate.

2. A method as defined by claim 1 wherein said titanium, said chromium, and said substrate are maintained at a temperature at about 900.degree. C.