US3244554A

US3244554A - Metal alloy plating process

Info

Publication number: US3244554A
Application number: US229726A
Authority: US
Inventors: Hamilton B Prestridge; Thomas P Whaley; Norman Vello
Original assignee: Ethyl Corp
Current assignee: Ethyl Corp
Priority date: 1962-10-10
Filing date: 1962-10-10
Publication date: 1966-04-05
Anticipated expiration: 1983-04-05

Description

Ari 5, 1966 PLATING GAS GENERATOR H. e. PRE STRIDGE ETAL METAL ALLOY ELATING PROCESS Filed Oct. 10, 1962 PLATING GAS GENERATOR PLATING CHAMBER United States Patent 0" 3,244,554 METAL ALLQY PLATENG PRQCESS Hamilton B. Prestridge, Baton Rouge, La, Thomas P.

Whaley, Skohie, IlL, and Velio Norman, (Ihapei ll-itili,

Nil, assignors to Ethyl Corporation, New York, N.Y.,

a corporation of Virginia Filed Get. 10, 1962, Ser. No. 229,726 6 Claims, (El. 117-107.2)

This invention relates to a process for producing unique metal alloy coatings on appropriate substrates by the art of vapor plating.

It is well known that Group VI-B metals viz. chr0- mium, molybdenum, and tungsten have very desirable physical properties which render them attractive for anticorrosive applications. For many applications, only a surface coating of these materials are employed since they are relatively expensive. For many applications, especially where a high degree of oxidation resistance is re quired, it has been observed that any alloy of these materials with aluminum produces a coating having excellent properties in this respect. The only successful approach reported in the literature to achieve this objective is a modified technique of the usual chromizing process. In that process, chromium-aluminum alloy coatings are formed by employing an atmosphere of the mixed chromium and aluminum trichloride vapors. While the results achieved by that process are beneficial for many applications, it invariably suffers the inherent disadvantages associated with the pack technique of metal plating.

The attendant disadvantages of the pack technique mentioned above are the necessity of using high temperatures, the lack of coating uniformity and the occurrence of simultaneous diffusion of the alloy coating into the substrate. The use of high temperatures restricts this technique to coating substrates which have not received special heat treatment during their fabrication which cannot thereafter be altered. The lack of coating uniformity associated with this technique is inherent in its procedure since the coating constituents which are packed in a container with the substrate to be coated can only be controlled by the temperature and time of the plating operation. Last but not least, the simultaneous difiusion that occurs during this operation results in an end product coating containing migrated substrate constituents which defeat the integrity and purpose of the coating for many applications.

Another approach taken to effect alloy coatings has been by diffusion of two or more superposed metal coatings. This approach has certain economic disadvantages, among others, since it requires two separate and distinct metal plating operations. Hence, a unified process whereby a Group VIB metal-aluminum alloy coating can be economically effected on a substrate without affecting its chemical and physical properties would represent a significant contribution to the art.

The above and other objects are accomplished by a process for effecting Group VI-B metal-aluminum alloy coatings at relatively low temperatures, significantly lower than heretofore employed in the art. A further object is to provide these coatings having a high degree of uniformity. Another object of this invention is to provide Group VIB metal-aluminum coatings having a high degree of adherence and integrity achieved without following an elaborate diffusion procedure. A more specific object is to provide these unique coatings upon substrates 3,244,554 Fatented Apr. 5, 1966 without altering the specific chemical and physical properties previously imparted to the substrate. Other objects will come to light as the discussion proceeds.

The above and other objects are accomplished by a process for the codeposition of the metals aluminum and a Group VI-B metal, viz. chromium, molybdenum, and tungsten upon a substrate which process comprises:

(1) heating said substrate to a temperature within the range of from about 150 C. to about 550 C. sufficient to thermally decompose the complex as hereinafter defined,

(2) contacting said heated substrate in an inert atmosphere with a plating gas comprising the vapors of an amine complex of aluminum hydride and the vapors of a Group Vl-B metal-carbonyl containing compound, and

(3) continually contacting said heated substrate with said vapors in said inert atmosphere until the desired thickness of coating is achieved.

Of the amine complexes of aluminum hydride, the tertiary alkyl amine complexes of aluminum hydride, especially the lower alkyl, that is those containing from 1 to 6 carbon atoms, are preferred since they are easier to prepare and offer an economic advantage. The trimethylamine and bis(t-rimethylamine) complexes of aluminum hydride as well as mixtures thereof, are highly preferred since they pyrolyze very smoothly and cleanly to deposit the highest purity aluminum coating in the shortest period of time. Of the Group VI-B metal-carbonyl containing compounds, the Group VIB metal hexacarbonyl compounds, i.e. Cr(CO) Mo(CO) and W(CO) are particularly preferred since they are more readily available and hence offer an economic advantage. The most preferred alloy coating effected by way of this invention is by codeposition of a trimethylamine complex of aluminum hydride and chromium hexaoarbonyl. When employing these two compounds, the highest purity alloy coating pursuant to this invention is realized.

A preferred operational sequence of conducting the above process comprises alternately contacting saidheated substrate first with the vapors of one compound for a brief interval of time and then with the vapors of the other compound for a brief interval of time. This procedure is continued until the desired thickness of metal alloy coating is achieved. If appropriate intervals of plating time are employed that are consistent with the deposition rates of the individual metals, an alloy coating comprising approximately equal percentages of both metals is realized. However, for most applications a preponderance of either one metal or the other is usually preferred. This objective is easily attained by following a plating sequence based upon a predetermined ratio of the plating times of the respective plating compounds consistent with the deposition rates of the individual metals as mentioned supra. In this embodiment it is preferred to contact the heated substrate with the metal plating vapors for relatively short periods of time. It has been found that best results are obtained when operating Within a time range of from about 5 seconds up to about 5 minutes. Thus, this embodiment comprises:

(1) heating said substrate to a temperature within the range of from about 150 C. to about 550 C. sufficient to thermally decompose the complex and the compound hereinafter defined,

(2) contacting said heated substrate in an inert atmosphere with the vapors of one of said metal-containing compounds for a brief interval of time.

(3) thereafter contacting said heated substrate in said inert atmosphere with the vapors of the other said metal-containing compound for a brief interval of time, and

(4) continually contacting said heated substrate in said inert atmosphere following the above sequence until the desired thickness of alloy coating is achieved.

Another preferred embodiment of this invention comprises contacting the heated substrate with a plating gas comprising mixed vapors of the aluminum-containing and Group VIB metal-containing compounds. In this em bodiment the composition of the effected alloy coating is primarily a function of the mass or preponderance of one compound in proportion to the other, consistent with the deposition rates of their individual metals. This embodiment comprises:

(1) heating a substrate to a temperature within the range of from about 150 C. to about 550 C. sufiicient to thermally decompose the complex and the compound hereinafter defined,

(2) pre-m-ixing the vapors of an amine complex of aluminum hydride and a Group VIB metalcanbonyl containing compound, and

(3) contacting said heated substrate in an inert atmosphere with said mixed vapors until the desired thickness of alloy coating is achieved.

Another unique embodiment of the instant invention comprises alternately regulating the temperature of the substrate simultaneous with-the contact of a specific plating agent as defined above. In other words, the substrate is maintained at one temperature while exposed to the vapors of, for instance, the aluminum-containing compound and thereafter adjusted to another temperature upon contact with the vapors of the Group VIB metalcarbonyl containing compound. When employing this technique, it is preferred to contact the heated substrate with the vapors of the amine complex of aluminum hydride when the substrate is at a temperature of from about 150 C. to about 250 C. On the other hand, when contacting the substrate with the vapors of a Group VIB metal-carbonyl containing compound it is preferred to do so when the substrate is at the temperature of from about 400 C. to about 500 C. Thus, this embodiment comprises:

( 1) maintaining a substrate at a temperature within the range of from about 150 C. to about 250 C. and contacting said heated substrate in an inert atmosphere with the vapors of an amine complex of aluminum hydride for a brief interval of time, and

(2) maintain said substrate at a temperature Within the range of from about 400 C. to about 500 C. and contacting said heated substrate in an inert at mosphere with the vapors of a Group VIB metalcarbonyl containing compound for a brief interval of time, said Group VIB metal being selected from the group consisting of chromium, molybdenum, and tungsten, and

(3) continuing the above alternate sequence of steps until the desired thickness of alloy coating is achieved.

The manner by which the above and other embodiments of this invention are practiced will be still further apparent from the drawing and ensuing description. In the drawing, the figure is a simplified process how dia gram illustrating means for accomplishing the preferred embodiments of the process of this invention. The amine complex of aluminum hydride and the Group VIB metal carbonyl compound may enter plating

gas generators

20 and 21 through feed lines and 11 respectively. Plating gases are generated in the plating gas generators by heat supplied by means not shown and carried directly and independently to plating chamber 22 by

feed lines

12 and 13. The amount of each plating gas being fed to plating chamber 22 may be controlled by

valves

25 and 26 located in

lines

12 and 13 respectively. It can be seen clearly that plating gas from each of the plating gas generators '29 and 21 may be fed to plating chamber 22 simultaneously in any ratio of one gas to the other or one at a time in an alternating sequence by suitable manipulation of valves 25 and 2t.

Another embodiment of the process of this invention encompasses having

valves

25 and 26 closed and premixing plating gases from plating generators 20 and 2 1 in line 14 by means of

feed lines

15 and 16 respectively. The amount of each plating gas being premixed in line 14 may be controlled and regulated by

valves

27 and 28. Line 16 indicates discharge from plating chamber 22 to provide a flow of plating gases through plating chamber 22 if needed or desired. A valve or other suitable control, pressure or vacuum means (not shown) may be provided in line '16, if desired.

Employing the classes of compounds disclosed herein in the unique process of this invention achieves unexpected and beneficial results. These results are unexpected in the light of the prior art teachings of various deficiencies associated with vapor plating utilizing heat decomposable metal-containing compounds. For reasons unknown, it has been found that by combining the use of the aboveidentified compounds in the process of this invention it is now possible to realize aluminum-Group VIB alloy coatings essentially free of undesirable carbides and oxides while simultaneously avoiding the problem of hydrogen embrittlement of metallic substrates. These results are also highly beneficial in that it is now possible to achieve a Group VI-B metal alloy coating, preferably chromium, having all the attendant desirable physical properties of the Group VIB metals enhanced by a higher degree of ductility achieved by the incorporation of a relatively small amount of aluminum. Conversely, it is now possible to achieve an aluminum alloy coating having improved wear resistance by the addition of a relatively small amount of a Group VIB metal, preferably chromium. Moreover, these unique coatings can be effected on substrates containing a high proportion of carbon in their composition and/or combined oxygen at its surface.

Throughout this specification and in the claims annexed to and forming a part of this specification, the term alloy is defined as a composition containing two or more metals wherein the average physical properties of the composite are different from those of the constituent metal. Thus, the term alloy may include intermetallics, solid solutions, or metal dispersions formed by diffusion as well as compositions wherein the nondispersed constituent metals are present as characteristic entities, but are in such intimate juxtaposition that the properties of one metal constituent are substantially influenced by those of the other metal. Looking at it another way, the alloys produced pursuant to this invention comply with recognized technical definitions of alloy. For example, in Hackhs Chemical Dictionary, 3rd Edition, 1944, McGraw-Hill Book Company, Inc., page 34, an alloy is defined as A mixture of two or more metallic elements which have a metallic appearance and which are either: (1) a molecular mixture, microscopically homogeneous; or (2) a colloidal mixture, microscopicallyheterogeneous.

The plating compounds to be employed in the process: of this invention are (1) Group VIB metal-carbonyl containing compounds and (2) amine complexes of alu-.

minum hydride.

The Group Vl-B metal-carbonyl containing compounds include (a) the Group VIB metal hexacarbonyls, namely chromium hexacarbonyl, molybdenum hexacarbonyl and, tungsten hexacarbonyl and (b) arene metal tricarbonyl compounds having the formula RM(CO) wherein R represents an aromatic group having a benzene or substituted benzene nucleus coordinated to a Group VI-B metal atom, M, through the carbon atom benzene ring, apparently by coordinate covalency. In other words, no hydrogen atom or substituted group has been displaced from the aromatic group in the creation of the bonding between the aromatic group and the metal atom. The carbonyl moieties of these arene Group VI-B metal carbonyls are also bonded to the metal atom, apparently by coordinate covalency. The donation of 6 electrons by the aromatic group and 6 electrons by the carbonyl moieties to the Group VI-B metal atom contributes materially to the stability of the molecule since the metal atom possesses the electron configuration of the next higher rare gas. Generally speaking these arene metal tricarbonyls contain an aromatic group which preferably contains from 6 to about 14 carbon atoms, the most preferred compounds containing from 7 to 9 carbon atoms in the arene group. Of the aromatic metal tricarbonyl compounds in the above formula, the most preferred are the alkyl substituted benzene metal tricarbonyls, such as xylene chromium tricarbonyl. Particularly useful compounds are o-xylene chromium tricarbonyl, p-xylene chromium tricarbonyl, m-xylene chromium tricarbonyl, toluene chromium tricarbonyl, and acetophenone chromium tricarbonyl. These compounds are more desirable since they possess optimum volatilities and thermal decomposition temperatures for use pursuant to this invention. Other suitable compounds are p-methyl anisole chromium tricarbonyl, mesitylene chromium tricarbonyl, mesitylene molybdenum tricarbonyl, mesitylene tungsten tricarbonyl, aniline molybdenum tricarbonyl, chlorobenzene molybdenum tricarbonyl, N,N- dimethylaniline molybdenum tricarbonyl, hexamethylbenzene tungsten tricarbonyl and the like.

Mixtures of the above compounds can also be employed, such as chromium hexacarbonyl and mesitylene chromium tricarbonyl, tungsten hexacarbonyl and benzene tungsten tricarbonyl, and the like. Mixtures of compounds having different metal constituents can also be employed, such as tungsten hexacarbonyl and molybdenum hexacarbonyl, benzene chromium tricarbonyl and mesitylene molybdenum tricarbonyl, chromium hexacarbonyl and ethylbenzene molybdenum tricarbonyl and the like.

Of the above Group VIB metal-carbonyl containing compounds, the hexacarbonyls are especially preferred since they are easily made at low cost and give very good results when used for this invention. From the standpoint of the Group Vl-B metals, the most attractive compounds are those containing chromium. It appears that the addition of a relatively small amount of aluminum to a chromium coating has the most pronounced eifect in improving its ductility as compared to the use of similar amounts of aluminum added to either of the other two Group VI-B metals.

The amine complex of aluminum hydride used in the practice of this invention is generally of two types: (1) amine complexes having only one amine molecule or constituent complexed with the aluminum hydride molecule, such as the trimethylamine complex of aluminum hydride, and (2) amine complexes wherein two amine molecules are complexed with the aluminum hydride molecule, such as bis-(trimethylamine) complex of aluminum hydride. The bis-amine complexes of aluminum hydride, especially the lower alkyl amine complexes of aluminum hydride, are preferred since they have very desirable volatility characteristics. By lower it is meant that the alkyl groups contain 6 carbon atoms or less for example, trimethylamine, triethylamine, tripropylamine, tributyiamine, trihexylamine, ethyldimethylamine, and the like. Trimethylamine complexes of aluminum hydride are highly preferred since coatings efiected by their use pursuant to this invention are characterized by high purity and adherence.

Exemplary of the amine complexes of aluminum hydride that are employed in the process of this invention are: triisopropylamine complex of aluminum hydride, tri-n-propylamine complex of aluminum hydride, pyridine complex of aluminum hydride, dimethylaniline complex of aluminum hydride, tetramethylethylene diamine complex of aluminum hydride, hexyltrimethyl ethylene diamine complex of aluminum hydride, bis-(trimethylamine) complex of aluminum hydride, bis-(triethylamine) complex of aluminum hydride, bis-pyridine complex of aluminum hydride, and the like. Mixtures of these compounds are also suitable, such as: trimethylamine complex of aluminum hydride and bis-(trimethylamine) complex of aluminum hydride, triethylamine complex of aluminum hydride and triisopropylamine complex of aluminum hydride, and the like.

A typical apparatus arrangement wherein the process of this invention can be conducted comprises a conventional plating chamber equipped with means for heating the substrate and provided with two vapor inlets and one vapor outlet. Connected to each inlet are separate standard vaporization chambers provided with individual heating means. Individual valve means are provided between the vaporization chamber and the heating chamber. Connected to the heating chamber outlet is vacuum inducing means. A typical plating operation comprises first placing the substrate to be plated within the heating chamber and thereafter evacuating the chamber atmosphere. The plating chamber or heating chamber is constantly maintained under vacuum throughout the plating operation. The substrate is then heated to an appropriate temperature within the range of from about 150 C. to about 550 C. The individual plating compounds contained in separate vaporization chambers are then heated to a temperature range of from about 0 C. to about C. to generate the desired vapor concentrations. When the substrate is up to plating temperature, the pressure in the plating chamber is adjusted to within a pressure range of from about 0.01 millimeter to about 10 millimeters of mercury.

To practice one embodiment of the instant invention the valves actuating the compounds from the heating chamber are alternately manipulated whereby the substrate is successfully exposed first to the vapors of one compound and then to the vapors of the other compound. Employing the above apparatus and the foregoing technique, the following examples further illustrate the conditions and results of this unique invention.

Example I Aluminum compound AlH (CH N. Compound temp 20 C. Time 10 seconds. Chromium compound Cr(CO) Compound temp 25 C. Time 10 seconds. Substrate Nickel. Pressure 0.1 mm. Total time 30 minutes.

Example II Aluminum compound AlH (CH N.

Compound temp 10 C.

Time 10 seconds.

Chromium compound Mesitylene chromium tricarbonyl.

Compound temp 120 C.

Time 30 seconds.

Substrate Mild steel.

Substrate temp 400 C.

Pressure 0.1 mm.

Total time 30 minutes.

7 Example III Aluminum compound AlH -2(CH N. Compound temp 25 C. Time seconds. r Chromium compound Cr(CO) Compound temp 25 C. Time 20 seconds. Substrate Nickel alloy. Substrate temp 325 C. Pressure 0.3 mm. 10 Total time 30 minutes.

Example IV Aluminum compound AlH -(C H N. Compound temp 35 C. Time 25 seconds. Molybdenum compound Mo(CO) Compound temp 30 C. Time 10 seconds. Substrate Cobalt alloy. Substrate temp 400 C. Pressure 0.1 mm. Total time 60 minutes.

Example V Aluminum compound AlH '(C H N. Compound temp 5 C. Time 10 seconds. Tungsten compound W(CO) Compound temp 35 C. Time 30 seconds. Substrate Mild steel. Substrate temp 375 C. Pressure 0.2 mm. Total time 90 minutes.

Example VI Aluminum compound Mixed vapors of AlH (CH N and AlH 2 (CH N. Compound temp 10 C. Time 30 seconds. Group VI-B metal cpds. Mixed vapors of W(CO) and xylene W(CO) Compound temp 100 C. Time 10 seconds. Substrate Nichrome. Substrate temp 300 C. Pressure 0.1 mm. Total time 150 minutes.

In the above examples, the appearance of the coatings could best be described as having a dull, silvery luster.

A visual inspection of the coatings reveals that they are essentially free of pits, blisters, pinholes, or other surface discontinuities. Adhesion of the metal to the substrate is inspected by means of a vertical indentation test. In general, all of the coatings exhibit excellent adherence, uniformity and ductility.

It is understood that the individual predetermined exposure time of the heated substrate to the respective vapors of the plating compound will determine the composition of the alloy coating in the foregoing embodiment.

Exposure times are predetermined by taking into consideration the deposition rate of a particular plating agent under the selected operating conditions. The deposition rate is a function of the selected operating conditions since the pressure and temperature employed during the plating will in turn determine the vapor concentration of the plating agent. For example, to achieve approximately an equal percentage of aluminum-chromium alloy coating, a preferred operational sequence calls for a 10 second alternate operation of the respective compound vapor control valve with appropriate adjustment of plating agent vapor pressures to compensate for the greatest deposition rate of the aluminum, as illustrated in Example I above.

As discussed above, another preferred embodiment of this invention calls for premixing the vapors of the respective plating compounds prior to their entry into the plating chamber. In this embodiment, if a carrier gas is not employed then the desired coating composition is a factor of the partial pressures or vapor concentrations of the individual vaporized plating compounds. In this instance, the plating rate is essentially equal to the decomposition rate of the individual compounds. However, for high production rates it is preferred to employ a carrier gas in this embodiment in which case cognizance must be taken of its efiect upon the partial pressure of the vapor concentration of the individual vaporized plating compound.

The above described apparatus is modified to the extent that the individual metal plating vapors are caused to be premixed before their contact with the heated substrate. The following example illustrates the conditions and results of this embodiment.

Example VII Compound temp 60 C Time 20 seconds. Substrate Copper. Substrate temp 400 C. Pressure 1 mm. Total time 2 /3 hours.

The coating obtained by way of the above example is characterized by a dull, silvery satin appearance. It exhibits excellent adherence, uniformity and ductility. The above embodiment can be varied to the extent that the plating compounds themselves can be premixed and subsequently heated in a common vaporization chamber, provided they are mixed in such a ratio that subsequent adjustment of the plating conditions permit the deposition of individual metals in the desired ratio. However, somewhat more care is necessary due to the greater possibility of error in the determination and control of the composition of the effected coating.

In the embodiment discussed above wherein the temperature of the substrate is varied commensurable with the most desirable decomposition temperature of a selected compound, the same apparatus employed in Examples I-VI can be employed. The following example illustrates this embodiment.

Example VIII Aluminum compound AlH -(C H N.

Compound temp 50 C. Time 4 minutes. Tungsten compound Tungsten hexacarbonyl W (CO) Compound temp 35 C. Time 1 minute. Substrate Mild steel. Substrate temp Al: 200 C.

W: 460 C. Pressure 0.1 mm. Total time minutes.

The above example produces a dull gray coating which has good adherence.

The substrate or object to be coated by the process of this invention is any substrate which is thermally stable at the conditions employed to effect decomposition of the plating agents employed herein. By thermal stability is meant the structural capability of withstanding the temperatures necessary to efiect decomposition of the plating agent. There are a diversity of substrates which can be utilized in the instant invention. Exemplary of these substrates are metals and alloys thereof, metalloids, ceramics, cermets, minerals, carbonaceous materials, paper, fibers, fabrics, glass, high temperature resistant plastics, and the like. The instant invention is particularly applicable to plating metallic substrates and their alloys since such substrates as a class are characterized by their high thermal stability. The most preferred of the metals are those of Groups III-B through VIII of the Periodic Chart of the Elements, Fisher Scientific Company, 1955, or alloys composed principally of one or more of such metals. These metals are:

Group IIIB: Group Vii-B:

Scandium Manganese Yttrium Technetium Lanthanum Rhenium Group IVB: Group VIII:

Titanium Iron Zirconium Cobalt Hafnium Nickel Group V-B: Ruthenium Vanadium Rhodium Niobium Palladium Tantalum Osmium Group VIB: Iridium Chromium Platinum Molybdenum Tungsten The temperature to which the substrate is heated to effect thermal decomposition of the plating compound is preferably within the range of from about 150 C. to about 150 C. to about 550 C. By decomposition temperature is meant that temperature necessary to eflfect a metal coating from a metal-containing compound employed herein. A preferred constant temperature range is from about 300 C. to about 450 C. since high purity coatings are effected within this range. In the embodiment wherein the temperature of the substrate is varied, it is preferred to contact the substrate with the aluminum-containing compound when it is at a temperature of from about 150 C. to about 250 C. and when contacting it with the chromium-containing compound, when it is at a temperature of from about 400 C. to about 500 C. Within these ranges the highest purity coatings are achieved.

Heating of the substrate can be accomplished by many well known means. Generally, resistance heating, infrared heating, or an induction heating means are preferred since they are particularly suitable for heating materials without contaminating them. The latter type of heating is especially attractive Where the temperature of the substrate is to be varied during the plating operation. Quite often, the nature of the substrate will influence the selection of a particular type of heating means. For instance, induction heating is preferred when working with small intricate substrates which do not possess a high thermal conductivity. Flat substrates, such as metal plates, are generally heated by conduction from resistance heating apparatus such as a hotplate. The determination of a suitable heating means is additionally influenced by the plating environment in relation to such factors as to whether the plating operation is a continuous or batch process, the number of substrates to be plated in unit time, and additionally to the physical and structural arrangement of the plating equipment. These factors are a matter of design Within the abilities of one skilled in the art.

For certain applications the substrate to be plated is preferably subjected to an initial preparation step. This is especially desirable in the case of metal substrates which are seldom commercially supplied free from contaminants. In other words, the degree of adherence achieved through this unique vapor plating process can in certain instances be further improved by appropriate metal surface pretreatment. The best metal surface preparation is achieved through degreasing with a solvent such as 1,1,2-trichloroethylene or the like followed by light sandblasting. It is well known that vapor plated coatings exhibit better adherence when they are effected on slightly uneven surfaces, such as those created by sandblasting as opposed to a highly polished surface. However, other suitable methods can be employed, such as acid pickling. On some substrates such as graphite and ceramics where the surface is already nonuniform, it is feasible only to degrease the surface preparatory to plating. Other well known methods of substrate pretreatment can be employed in lieu of the above.

It is extremely desirable to maintain an inert atmosphere during the plating operation, in absence of which impure or oxidized coatings will be realized. An inert atmosphere can be provided in a number of ways. One way is to continually purge the plating system with an inert gas which is not compatible with the substrate, the plating compound and effected coating. Examples of suitable inert gases are carbon monoxide, nitrogen, hydrogen, helium, neon, argon, krypton, Xenon, gaseous aliphatic hydrocarbons and the like. Where such an inert medium is employed, it becomes particularly attractive to employ a carrier gas as a means of transporting the vapors of the plating compounds to the surface of the substrate since the same inert gaseous medium can be employed to perform the same functions, either concurrently or separately. Another suitable method to proide an inert atmosphere is by conducting this process under a reduced pressure. By evacuating the plating system undesirable contaminants will be removed. Additionally, the use of a reduced pressure during the plating operation will assure eificient and expeditious removal of the by-products of decomposition. Low pressure operation also insures a fairly constant vapor fiow rate. To take advantage of the attractive features of the above methods of providing an inert atmosphere, it is preferred to utilize a carrier gas in conjunction with vaporization and transportation of the plating compound while simultaneously and continually producing a vacuum on the plating system. This technique is preferred since it in sures a high degree of throwing power of the plating compound and also provides for the rapid removal of the byproducts of decomposition. The carrier gas is preferably provided by dissolving the plating compound in an organic solvent, such as hydrocarbon solvents, for example, alkanes, aromatics, cycloalkanes, fused ring aromatics, and the like, whereby the carrier gas and the desired plating vapors are generated simultaneously. This procedure offers an economic advantage in that it minimizes equipment and operation cost. When employing reduced pressures, it is preferred to conduct this process at a pressure less than 10 millimeters of mercury absolute. Operating within this pressure range assures rapid removal of contaminants that invariably leak into the system and also assures desirable flow rates of the plating compound.

Flow rates of the plating vapors employed in the process of this invention are not critical. In terms of mass flow, the primary consideration is the desired production rate. For most commercial applications a mass flow within the range of from about 1 gram to about 60 grams per hour per square inch of substrate surface is suitable. The weight in grams represents the weight of the metallic constituent of the plating compound which in essence represents the effected coating less nominal losses. A preferred mass how of the aluminum-containing compound is with the range of from about 3 grams to about 12 grams of aluminum per hour per square inch of substrate surface. In the case of the Group VI-B metalcontaining compound, a preferred range is from about 12 to about 60 grams of the Group VIB metal per hour per square inch of substrate surface. These flow rates are preferred since an economical utilization of the respective plating compounds are realized. The flow rate of the plating vapors is conveniently controlled by heating the compound, care being taken not to exceed its decomposition temperature, relative to the system pressure employed. When working with the preferred compound disclosed supra it is preferred to beat them to a temperature range of from about 20 C. to about 100 C. since they are easily volatilized within this range at pressures less than millimeters whereby convenient flow rates are possible.

The velocity flow rate of the plating gas is also not critical, being to a large extent dependent upon the size of the plating apparatus and the desired production rate. A suitable velocity range is anywhere from about 0.3 meters per second to about 120 meters per second, although, faster rates can be employed. Velocity of the plating gas is measured relative to the substrate surface or can be defined as the velocity of impingement. At extremely low velocities a coating exhibiting desirable mechanical properties is obtained but is sometimes discolored. At extremely high velocities a considerable amount of the plating atmosphere passes over the substrate surface without decomposing which adds to the expense of the process.

The time required to plate a substrate by the process of this invention varies over a wide range governed primarily by the specific product to be produced, that is the surface area and thickness of coating desired, and the size of the plating operation. For most commercial applications times from about minutes to about 10 hours are generally suitable.

Pursuant to this invention it is possible to achieve a Group VI-B metal coating having a high degree of ductility by incorporating a desired amount of aluminum into the coating. For example, chromium coatings are highly desirable for applications requiring a hard wearing and/or corrosion resistant material. Typical applications demanding such characteristics are pump shaft sleeves, valve stems, fasteners, aircraft parts, in short many applications demanding a chromium coating which suffers the deficiencies of the prior art associated with chromium coatings in that they are characterized by lack of ductility. The same comment applies to molybdenum and tungsten coatings which heretofore have been notoriously known as lacking ductility, particularly when subjected to a high temperature environment where the problem is accentuated. On the other hand, by way of the instant invention it is possible to achieve an aluminum coating having a high degree of wear resistance by the addition of a relatively small amount of a Group VI-B metal. Aluminum coatings find widespread use as a corrosion resistant material for such applications as pump parts, hardware and parts of internal combustion engines, and the like. Aluminum coatings are also attractive for applications requiring an oxidation resistant material, such as turbine vane blades. Although not necessary, the adherency of a coating effected by way of this invention can generally be improved by subjecting the coated substrate to a subsequent diffusion step for certain applications, particularly those where high temperatures are encountered. Where desired, diffusion of a coating can be effected by heating the coated specimen between 1200 F. to 1950 F. for times from 1 to 16 hours, preferably in an inert atmosphere such as hydrogen, nitrogen, and the like to prevent oxidation and contamination of the coating during diffusion.

We have fully described the novel process of the present invention, and we do not intend that our invention be limited except within the spirit and scope of the appended claims.

What is claimed is:

1. A process for the codeposition of the metals aluminum and a Group VI-B metal comprising (l) heating a substrate to a temperature within the range of from about C. to about 550 C. sufficient to thermally decompose the plating gas hereinafter defined,

(2) contacting said heated substrate in an inert atmosphere with a plating gas comprising the vapors of an amine complex of aluminum hydride and the vapors of a Group VL-B met-alcarbonyl containing compound, said Group VI-B metal being selected from the group consisting of chromium, molybdenum, and tungsten, and

2. The process of claim 1 further characterized in that said amine complex of aluminum hydride is a tertiary amine complex of aluminum hydride wherein each alkyl group of the amine contains from 1 to 6 carbon atoms and in that said Group VI-B metal-carbonyl containing compound is selected from the group consisting of chromium hexacarbonyl, molybdenum hexacarbonyl, and tungsten hexaca rbonyl.

3. The process of claim 1 further characterized in that said substrate is heated to a temperature range of from 300 C. to about 450 C. and said amine complex of aluminum hydride is a trimethylarnine complex of aluminum hydride and said Group VI-B metal-carbonyl containing compound is chromium hexacarbonyl.

4. A process for the codeposition of the metals aluminum and a Group VI-B metal comprising (1) heating a substrate to a temperature within the range of from about 150 C. to about 550 C. suflicient to thermally decompose the complex and the compound as hereinafter defined,

(2) alternately contacting said heated substrate in an inert atmosphere with the vapors of an amine complex of aluminum hydride for a brief interval of time and then withthe vapors of a Group VI-B metal-carbonyl containing compound for a brief interval of time, said Group VIB metal being selected from the group consisting of chromium, molybdenum, and tungsten, and

(3) continually contacting said heated substrate in said inert atmosphere following the above alternate sequence until the desired thickness of alloy coating is achieved.

5. A process for the codeposition of the metals aluminum and a Group VI-B metal comprising 1) heating a substrate to a temperature within the range of from about 150 C. to about 550 C. sulficient to thermally decompose the complex and the compound as hereinafter defined,

(2) premixing the vapors of an amine complex of aluminum hydride and a Group VI-B metal-carbonyl containing compound, said Group VI-B metal being selected from the groupconsisting of chromium, molybdenum, and tungsten, and

6. A process for the codeposition of the metals aluminum and a Group VI-B metal comprising alternating between the steps of (1) maintaining a substrate at a temperature within the range of from about 150 C. to about 250 C. and contacting said heated substrate in an inert atmosphere with the vapors of an amine complex of aluminum hydride for a brief interval of time, and then (2) maintaining said substrate at a temperature within the range of from about 400 C. to about 500 C. and contacting said heated substrate in an inert atmosphere with the vapors of a Group VI-B metalcarbonyl containing compound for a brief interval of time, said Group VI-B metal being selected from the group consisting of chromium, molybdenum,

and tungsten, and

References Cited by the Examiner UNITED STATES PATENTS 14 FOREIGN PATENTS 915,385 1/1963 Great Britain.

OTHER REFERENCES 5 Wiberg et aL: Zeitschrift fur Anorganische und Allgemeine Chemie, vol. 272, pp. 221 and 226, 1953.

McMannuS 117 107 RICHARD D. NEVIUS, Primary Examiner. Drummond 117-1072 WILLIAM D. MARTIN, JOSEPH B1 SPENCER, Gurinsky 1171 07.2 10 Examiners.

Whaley 117-107.2

Claims

1. A PROCESS FOR THE CODEPOSITION OF THE METALS ALUMINUM AND A GROUP VI-B METAL COMPRISING (1) HEATING A SUBSTRATE TO A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 150*C. TO ABOUT 550*C. SUFFICIENT TO THERMALLY DECOMPOSE THE PLATING GAS HEREINAFTER DEFINED, (2) CONTACTING SAID HEATED SUBSTRATE IN AN INERT ATMOSPHERE WITH A PLATING GAS COMPRISING THE VAPORS OF AN AMINE COMPLEX OF ALUMINUM HYDRIDE AND THE VAPORS OF A GROUP VI-B METAL-CARBONYL CONTAINING COMPOUND, SAID GROUP VI-B METAL BEING SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN, AND (3) CONTINUALLY CONTACTING SAID HEATED SUBSTRATE WITH SAID VAPORS IN SAID INERT ATMOSPHERE UNTIL THE DESIRED THICKNESS OF COATING IS ACHIEVED.