CA1245111A - Process for applying hard coatings and the like to metals and resulting product - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

Hard coatings are applied to substrate metals by coating the metal surface, e.g. by dipping the substrate metal in a molten alloy of the coating metals, and then exposing the coating at an elevated temperature to an atmosphere containing a reactive gaseous species which forms a nitride, a carbide, a boride or a silicide. The coating material is a mixture of the metals M1 and M2 of which M1 forms a stable nitride, carbide, boride or silicide under the prevailing conditions and of which M2 does not form a stable nitride, carbide, boride or silicide. M2 serves to bond the nitride, etc. of M1 to the substrate metal.
Mixtures of M1 and/or M2 metals may be employed. This method is much easier to carry out than prior methods.

Description

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~PROCESS FOR APPLYING HARD COATINGS
AND THE LIRE ~0 METALS AND RESULTING PRODUCT~

This invention relates to the coating of Ketals (hereinafter referred to as ~substratesU or ~substrate metals~) with co~tings that serve to provide hard surfaces, chemically res~stant coatings, etc.

Hard coatings were developed for the purpose of providing a combination of high performance properties ~uch as resistance to friction, wear and corrosion to less expensive metal component~. Early techniques used in the application of these coatings were based on surface treatment of metallic ~ubstrates by the diffusion of carbon, nitrogen, boron, or silicon, thus generating the hard material~ ~irectly in the ~urface of the ~ubstrate. Most of the more recent application techniques involve the deposition of ~n overlay hard layer as Rn external coating.
Examples of techniques include- Chemical v~por deposition ~CVD), physical vapor deposition ~PYD), laser fusion, sputtering, flame or plasma 6pr~ying, and de~onation gun.
With the possible exception of C~D processes, these techniques are expen~ive and limited to the line of ~ight which may lead to ~ariable thickness and unequal cover2ge particularly at corners, holes and complex shapes.
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It is an ob~ect of the present invention to provide an improved method of applying to substrate metals coatings of MlXn where Ml is the metal whose compound is to b~ applied to the substrate, X is an element such as nitrogen, carbon, boron or silicon, and n is a number indicating the atomic proportions of X to M.

It is a further object of the invention to provide coated substrate metals in which the coatings, MlXn as described above, are uniform and adherent to the substrate.

The above and other objects of the invention will be apparent from the ensuing description and the appended claims.

The invention provides a method of coating a metal substrate with a protective coating which comprises: (a) providing a substrate metal to be coated, (b) providing an alloy or mixture of at least one metal Ml, and at least one other metal M2 selected according to the following criteria:
(1) Ml is susceptible to reaction with a reactive gaseous species of an element X tX being nitrogen, carbon, boron or silicon) to form a stable compound of Ml and X at a selected temperature and pressure of such reactive species, (2) M2 does not form a stable compound with X under such conditions and it bonds to the substrate on heat treatment of the coated material;
(c) applying such alloy or mixture to a surface of the substrate to provide a coating and (d) effecting selective reaction of M
with such gaseous species at an elevated temperature under conditions to produce a compound of Ml and X and to avoid or minimize formation of a compound of M2 with X.

The invention further provides a coated metal article comprising~ (a) a metal substrate and Ib) a protective coating on and adherent to at least one surface of the metal substrate, such coating comprising an outer layer of a compound MlXn ~. .

- 2a -wherein X is nitrogen, carbon, boron or silicon and n represents the atomic proportion of X to M1 and an inner layer of at least one metal M2 bonded to the substrate, said metals Ml and M2 being selected according to the following criteria: (1) Ml is susceptible to reaction with a reactive gaseous species of an element X (X being nitrogen, carbon, boron or silicon) to form a stable compound of M1 and X at a selected temperature and pressure of such reactive species, (2) M2 does not form a stable compound with X under such conditions and it bonds the coating to the substrate.

In accordance with the present invention, an alloy or a physical mixture of metals is provided comprising two metals Ml and M2 which are selected in accordance with the criteria described below. This alloy or metal mixture is then melted toprovide a uniform melt which is then applied to a metal substrate by dipping the substrate into the melt.
Alternatively, the metal mixture or alloy is reduced to a finely divided state, and the finely divided metal is incorporated in a volatile solvent to form a slurry which is applied to the metal substrate by spraying or brushing.
The resulting coating is heated in an inert atmosphere to accomplish evaporation of the volatile solvent and the fusing of the alloy or metal mixture onto the surface of the substrate. (Where physical mixtures of metals are used, they are converted to an alloy by melting or they are alloyed or fused together in situ as in the slurry method of application described above.) In certain instances, as where the alloy melts at a high temperature such that the substrate metal might be adversely affected by melting a coating of alloy, the alloy may be applied by plasma spraying.

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The metals Ml and M2 are 6elected according to the following criteria: Ml ~orms ~ thermally fitable compound with X (i.e., a nitride, n carbide, a boride or ~ ~ilicide) when exposed at a high temperature to an ~tmosphere csntaining a ~mall concentr~tion of X or of a dissociable molecule or ~ompound of X. The 6table compound that Ml forms with X may be repre ented as MlXn where n represents the atomic ratio of X to Ml.

~ he metal M2, under such conditions, does not form a stable compound with X and remains entirely or ~ubstantially entirely in metallic form. Further, M2 is compatible with the substrate metal in the ~ense that it results in an intermediate layer between the MlXn outer layer (resulting from reaction with X) and the substrate, ~uch intermediate layer 6erving to bond the MlXn layer to the substrate. Interdiffusion of M2 and the substrate metal aids in this bonding effect.

It will be understood that Ml may be a mixture or alloy of two or more metals meeting the requirements of M
and that M2 may Also be a mixture or alloy of two or more metals meeting the requirements of M2.

The coating thus formed and applied i~ then preferably subjected to an annealing step. The ~nnealing step may be omitted when annealing occurs under conditions of use.

When ~ coating of suitable thickness has been applied to the substrate alloy by the dip coating process or by the slurry process described above ~and in the latter case after the solvent has been evaporated and the M/M2 metal alloy or mixture is fused onto the surface of tbe substrate) or by any other suitable process the surface is then exposed to a selectively reactive atmosphere at an appropriate elevated temperature.

~ o form ~ nitride, carbide, boride or ~ilicide layer on t~e substrate metal, ~n appropriate, thermally di~soci~ble c~pound or molecule of nitrogen, carbon, boron or ~ilic~n may be used. Examples ~f ~uitable gaseous media ~re ~et forth in Table ~ below $ncluding ~edia where X
nitrogen, etc.

Table I. Gaseous Media for Forming Nitrides, Carbides, - Borides and Silicides X Gaseous Media N N2, NH3 or ~ixtures of the two.

C Methane, ~cetylene.

B Borane, diborane, borohalides.

Si Silane, trichloro silane, tribromosilane, silicon tetrachloride.

Where a very low partial pressure of the reactive species is needed, that species may be diluted by an inert gas, e.g. argon . If the RCtive species results from a gaseous reaction of two precursor species, the concentration of the active species may be controlled by adjusting the ratio of the precur~or ~pecies.

~ here results from this process a structure such as shown in Figure 1 of the drawin~s.

Referring now to Pigure 1, this ~igure represents a cross-section through a substrate alloy indicated at 10 coated with a laminar coating indicated at 11. -The laminar coating 11 consists of an intermediate metallic layer 12 and an outer MlXn layer 13. The relative thicknesses of the layers 12 and 13 are exaggerated. The substrate layer 10 is as thick as required for the intended service.

The layer6 12 and 13 together typically will be about 300 to 400 microns thick, the layer 12 will be about 250 microns thick, and the layer 13 will be about 150 micron~ thick. It will be understood that,the layer 12 will have a thickness adequate to form a firm bond with the substrate and that the layer 13 will have a thickness ~uiting it to its intended use.

Figure 1 is a simplified representation of the coating and substrate. A more accurate representation is 6hown in Pigure lA in which the ~ubstrate 10 and outer layer MlXn are as described in Figure 1. However there is a diffusion zone D which may be an alloy of one or more substrate metals and the metal M2 or it may be an inter-diffusion layer resulting from diffusion of substrate metal outwardly away from the ~ubstrate and of M2 inwardly into the 6ubstrate. There i~ al80 an intermediate zone I which may be a cermet formed as a composite of MlXn and M2.

The metals Ml and M2 will be se~ected according to the intended use. Table II below lists metals which may be used as Ml and Table III lists metals that may be used as M2. Not every metal in Table II may be used with every metal in Table IIIS it is required that M2 be more noble than Ml in any Ml/M2 pair. Another factor is the intended use, e.g. whether a hard sur~ace, or a ~urface which i5 resistant to ~queous environments lc desired, ~ surface which acts ns a lubricant, etc. Also the n~ture of the substrate should be considered. It will be ~een that some ~etals appear in both tables; that i~ a metal Ml appearing in Table II ~ay be used a~ M2 (the more noble ~etal) with a less noble metal Ml from Table III.

Table $I (Ml~

Actinium Neodymium Aluminum Niobium Larium Praseodymium~
Beryllium Samarium Calcium Scandium Cerium Silicon Chromium Tantalum Dysprosium Terbium Erbium Thorium Europium Thulium Gadolinium Titanium ~afnium Tungsten Holmium Van~dium Lanthanum Ytterbium Lithium Yttrium Magnesium Zirconium Molybdenum ~5~

Table III (M2) Cobal~ Palladium Copper Platinum Gold Rhenium Iridium Rhodium Iron Rubidium Manganese Ruthenium Molybenum Silver Nickel ~in Osmium Zinc It will be understood that two or more metal chosen from Table II ~nd two or more metal~ chosen $rom ~able III may be employed to form the coating alloy or mixture. Examples of suitable Ml/M2 metal pairs including mixtures of two or more metals Ml and two or more metals M2 are fiet forth in Table IV.

~able IV

Ml M2 ~51 kl2 _ . . . .
Ti Ni Th Ni ~i Fe ~h Pe Ti Co ~h Co Ti Cu Th Mg Ti Pd Ti + Nb Ni Ti ~ Zr Co Th . Cu Ti + Zr Fe Th Al Ti ~ Zr Cu Sc Al Zr Fe Zr Cc~ Sc Cu Zr Cu Sc Fe Zr Pd Zr Pt Sc Pd Zr Rh Sc Ru Zr + Y Ni Y Al Zr + Y Co Y Co Zr ~ Y Fe Zr + Y Pd y S::u y Fe Zr + Nb ~ Ni Zr + Hf Ni Y Ni Hf N i : Y Pd CU Y ~1 Si Nb Si Co Si ~e Si Mo 5i Ni Si Pd Si Pt Sr Ni Cr Pd Si Ru ~S~ll It will be under~t~d that not every metal pair will be 6uitable for all purposes. ~or example, where Ml is silicon the coating tends to be brit~1e; 6~me p~ir& ~re better ~uited for hardness,others for s~rvice as ther~31 barriers, others for oxidat~on ~nd corro~i~n resi~tance, etc.

Examples o eutectic ~lloy~ ~re li~ted in Table V.
~t will be understood that not ~11 of the~e alloys.~re useful on ~ ubs~rate6. In some cases the melting points are apprcximate. Numbers indicate the approximate percentage by weight of H2.

Table V

Eutectic Allov Melting Point ~C) Ti - 28.5 Ni 942 Ti - 32 Fe 1085 Ti - 2B Co 1025 Ti - 50 Cu 955 Ti - 72 Cu 885 ~i - 48 Pd 1080 Zr - 17 Ni 960 Zr - 27 Ni 1010 Zr - 16 Fe 934 Zr - 27 C~ 1061 Zr - 54 Cu B85 Zr - 27 Pd 1030 Zr - 37 Pt 1185 Zr 25 Rh 1065 ~f - 72 Ni 1130 Hf - 38 Cu 970 Th - 36 Ni 1037 .
~h - 17 Fe 875 Table V ~t:ont 'd. ~

Eutectic AllovMelting Point (C) Th - 30 Co 975 ~h - 22. 5 Cu 880 Th - 75 Al 632 Sc - 45 Al 1150 S~ - 77 Cu 875 Sc - 24 Fe 910 Sc - 22 Pd 1000 ;~c ~ 17 Ru 1100 Y - 93 Al 64û
Y - 19 Al 1100 Y - 9. 5 Al 960 Y - 28 Co 725 Y - 88 Cu 890 Y - 66 Cu 840 Y - 50 Cu B30 Y - 27 Cu 760 Y - 25 Fe 900 y _ 47 Ni 9 Y - 25 Ni 802 Y - 34 Pd 903 Y - 28 Pd 907 Y - 17 Ru 1080 Nb - 76. 5 Ni 1270 Nb D 48. 4 Ni 1.175 Si - 88. 3 Al 577 Si - 37. B Co 1259 Si - 8~ Cu 802 Si - 42 Fe 1200 Si 12 Mo 1410 Si -- 62 Ni 964 Si - 74 Pd B70 Si - 77 Pt 979 Table VA li~ts ~rtain tertiary ~lloys that are useful in the practice o~ the present invention.

Table VA

55.18 ~i - 23.13 Nb - 21.69 Ni 40.38 Ti - 43.52 Zr - 16.10 Ni ~0.07 ~i - 44.35 Zr - 15.58 Co 25.37 Ti - 65~69 Zr - 11.94 Fe 17.36 ~i - 38.01 Zr - 44.63 Cu 69.65 Zr - 16.07 Y ~ 14.26 Ni 55.96 Zr - 23.34 Y - 20.70 Ni 43.08 Zr - ~0.98 Y ~ 15.94 ~o 56.76 Zr 32.43 Y - 10.81 Fe 47.B9 Zr - 34.39 Y 17.72 Pd S6.68 2r - 22.35 Nb 20.97 Ni 49.33 Zr - 32.43 Hf - 43.94 Ni 24.20 Zr - 48.51 Hf - 27.29 Ni Yttrium, calcium and magnesium are especially beneficial in zirconium-noble metal ~M2) alloys because they stabilize zirconia in the cubic form. Examples of such ternary alloys are as follows.

Zr Y Ca Mg Ni 77 7 1~

Table VI provides examples of metal substrates to which the metal pairs may be applied.

Table YI

Superalloys Cast nickel base such ~s IN 738 Cast cob~lt base ~uch n~ MAR-M509 Wrought nickel ba~e 6uch ~s RRné 95 Wroughe cobalt b~se such ~s ~aynes ~lloy No. lB8 Wrought iron base ~uch ~s Discaloy Hastalloy X

Incoloy*901 Coated superalloYs (coated ~or corrosion resistance) Superalloys coated with Co(or Ni)-Cr-Al-Y alloy, e.g. 15-25~ Cr, 10-15~ Al, 0.5~ Y, balance is Co or Ni Steels ~ool Steels (wrought, cast or powder metallurgy) ~uch as AISIM2; AISIWl Stainless Steels Austenitic 304 Ferritic 430 Martensitic 410 Carbon Steels AlloY Steels Maraging 250 * trade mark .~

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C~-t irons Gray, ductile, malleable, alloy Non-ferrous Metals ~itanium and titanium alloys, e.g. ASTM Grade l;
Ti-6Al-4V
Nickel and nickel alloys, e.g. nickel 200, Monel 400 Cobalt Copper and its ~lloys, e.g. C 10100; C 17200;
C 26000; C 952C0 .
Refractory metals and allovs Molybdenum alloys, e.g. TZM
Niobium alloys, e.g. FS-85 Tantalum alloys, e.g. T-lll Tungsten alloys, e.g. W-Mo alloys Cemented Carbides Ni and cobalt bonded carbides, e.g. WC-3 to 25 Co Steel bonded carbides, e.g. 40-55 vol.S TiC, balance steel; 10-20~ TiC-balance ~teel The proport$ons of Ml to M2 may vary widely depending upon ~uch factors as the choice of Ml and M2, the nature of the ~ubstrate metal, the choice of the reactive gaseous species, the conversion temperature, the purpose of the coating (e.g. whether $t is to serve as a thermal barrier or as a hardened surface), etc.

~2451~L1 The dip coating method is pre~erred. I~ is easy to carry out and the molten ~lloy Femoves 6urface oxides (which tend to cause Epallation)~ In thi~ ~ethod ~ molten Ml/M2 ~lloy ~ provided and the ~ubstrate alloy i8 dipped into a body of the coatlng alloy. The temperature of the ~lloy ~nd the time dur~ng which the substrate is held in the molten alloy will control the thickness ~nd smoothness of the coating. If an ~erodynamic ~urface or ~ cutting edge ~8 being prepared ~ ~moother rurface will be desired than for some other purposes. ~he thickness of the applied coating can range between a fraction of one micron to a few millimeter~. Preferably, a coating of about 300 microns to 400 microns is ~pplied if the purpose i8 to provide a thermal b~rrier. A hardened ~urface need not be ~s thick.
It will be understood that the thickness of the coa~ing will be provided in accordance with the requirement6 of a particular end use.

The slurry fusion method has the advantage that it dilutes the coating alloy or metal mixture and therefore makes it possible to effect better control over the thickness of coating ~pplied to the ~ubstrate. Also complex ~hapes cDn be coated and the proGess can be repeated to build up a coating of desired thickness. Typically, the slurry coating technique may be applied ~s follows: A
powdered alloy of Ml ~nd M2 is mixed with ~ mineral ~pirit and an organic cement ~uch as Nicrobraz 500* (Well Colmonoy Corp.) and MPA-60 (Baker Caster Oil Co.). Typical proportions u~ed ln the slurry are coating alloy 45 weight percent, mineral ~pirit 10 weight percent, ~nd organic cement, ~S weight percent. This mixture is then ground, for example, in ~ ceramic ball mill using aluminum oxide * trade mark : s"

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balls. After separation of the resulting 61urry from the al~mina ball~, it ~ applied IkeePing ~t ~tirred to insure uniform dispersion of the particle~ of alloy ln the liquid medium) to the 6ubstrate surface and the solvent i~
evaporated, for example, ~n a$r at amb$ent temperature or at a ~omewhat elevated temper~ture. The residue of alloy and cement i8 then fused onto the surface by heating ~t to a suit~ble temperature $n an ~nert atmosphere ~uch as argon that has been passed over bot calcium chips to getter oxygen. The cement will be decomposed and the products of decomposition are volatilized.

If the alloy of Ml and M2 h~s ~ melting point which i~ suf f iciently high that it exceeds or closely approaches the melting point of the substrate, it may be ~pplied by qputtering, by vapor deposition or some other technique.

It is advantageous to employ Ml and M2 in the form of an alloy which is a eutectic or near eutectic mixture. This has the advantage that a coating of definite, predictable composition is uniformly applied. Also eutectic and near eutectic mixtures have lower melting points than non-eutectic mixtures. Therefore they are less likely than high melting alloys to harm the substrate metal and they sinter more readily than high melting alloys.

~ he following specific examples will serve further to illustrate the practice and advantages of the invention.

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Example 1~

The substrate metal was ~ool ~teel in the form of rod. The coating ~lloy was a eutect$c ~lloy containing 71.5~ nd 28.5~ Ni. Thifi eutectic has a melting point of 942C. The rod wa~ dipp~d ~nto this alloy at 1000C ~or 10 ~econds ~nd was removed and annealed or 5 hourR ~t 800C. It was then exposed to oxygen free nitrogen for 15 hours at 800C. ~he nitrogen was passed slowly over the rod at atmospher~c pressure. The resulting coating was continuous ~nd adherent. The composition of the titanium nitride, TiNX, depends upon the temperature and the nitrogen pressure.

Example 2.

Example 1 was repeated using mild steel as the substrate. A titanium nitride layer was applied.

The coatings of Examples 1 and 2 are useful because the treated surface is hard. This is especially helpful with mild steel which ~s inexpensive but soft.
This provides a way of providing an inexpensive metal with a hard ~urface.

Example 3.

The same procedure was carried out as in Example 1 but at 650~C. The coating, 2 microns thick, was lighter in color than the coating of Exampie 1.

Darker colors obtained at higher temperatures indicated a ~oichiometric composition; TiN.

Similar coatings were applied to stainless steel.

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Example 4.

A eutectic ~lloy of B3~ Zr ~nd 17~ Ni (melting point ~ 961C) is employed. ~he ~ubstrate ~etal (tool steel) is dip coated at 1000C, ~nnealed 3 hours ~t 1000C
and exposed to nitrogen as $n Exampl~s 1 ~nd 3 at 800C.
A uniform ~dherent coating 2 to 3 ~icron~ thick resulted.

Example 5.

~ ~84 Zr - 52% Cu eutectic alloy, ~elting point 885DC was used. ~ool steel was dipped into the ~lloy for 10 ~econds at 1000C and was withdrawn ~n~ annealed-5 hours ~t lOOO~C. It was then exposed to nitrogen ~t one atmosphere for 50 hours at 800C. A uniform adherent coatlng resulted.

An advantage of copper ~s the metal M2 is that it is a good heat conductor which is helpful in carrying away heat (into the body of the tool) in cutting.

Example 6.

A 77% Ti - 23~ Cu ~lloy, a eutectic alloy, melting at 875C was used. Hot dipping was at 1027C for 10 seconds; anne~ling at 900C for S hours; exposure to N2 at 900C for 100 hours. An adherent continuous coating resulted. The ~ubstrate metal was high speed steel.

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Tool steel was coated with a Ti-Ni ~lloy and annealed as in Example 3. ~he reactive 9a5 species is methane which may be used with or without ~n inert g2s diluent Euch as argon or helium. The co~ted steel rod is expo~ed to methane at 1000C ~or 20 hours~ A hard, adherent coating of tit~nium carbide result~.

Example 8.

The procedure of Example 7 may be repeated using BH3 as the reactive gas species ~t ~ temperature above 700C, e.g. >700C to 1000C, for ten to twenty hours.
A tit~nium boride coating i8 formed which i6 hard ~nd adherent.

Example 9.

The procedure of Example 7 is repeated using silane, Si H4, as the reactive gas Epecies, with or without a diluting inert gas such as argon or helium.
The temperature and time of exposure may be >700C to 1000C ~or ten to twenty hours. A titanium silicide coating is ~ormed which is hard and adherent.

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--lg-Among other con6ider~tions ~re ~he ollowing:

The metal M2 ~hould be compatible with the ~ub~trate. For example, ~t ~hould not form brittle inter-me~allic compound wieh metal~ of ~he ~ub~tr~te. Prefer~bly it doe~ nvt alter Eeriou~ly the mechanical pr~per~ie~ ~f the 6ubstrate ~nd ha~ a large rDnge of ~olid ~olubility in the substrate. Al~o $t prefer~bly ~orm~ ~ low nelt~ng eutectic with Ml. Al~o $t should not form a highly stable carbide, nitride, boride or æilicide. ~or example, if Ml ls to be converted to a nitride, M2 ~hould not form a ~table nitride under the conditions employed to form the ~1 nitride. -In the hot dipping method of application of anMl/M2 alloy, uneven ~urface application may be ~voided or diminished by ~pinning and/or wiping.

~ he annealing ~tep after application of the alloy or mixture of Ml and M2 ~hould be carried out to 6ecure a good bond between the alloy and the substrate.

Convérsion of the ~lloy coat$ng to the final product i~ preferably carried out by expo~ure to a ~lowly flowing stream of the react$ve gas at a temperature and pressure ~ufficient to react the reactive gaseous molecule or compound with Ml but not such as to react with M2.
It is also advantageous to employ a temperature slightly above the melting point of the ~oat$ng ~lloy, e.g. El$ghtly ~bove its eutestic melting pGint. ~he presence of a liquid phase promote6 migration of Ml to the ~urface ~nd di~placement of M2 in the outer layer~

If the temperature i~ below the melting point of the coa~ing alloy and ~f the compound ~ormed by Ml and the reactive gaseous ~pecies grQws ast, M2 will be entrapped in the growing compound, thus bonding the particles of MlXn. In this case ~ cermet will Ibe formed which ~ay t~e ~dvant~gec~us, e.g. ~ W or Nb carb~de cemented by cob~lt or n ickel .

It will therefore be apparent ~h~ new ~nd useful method of ~pplying MlXn co~ting to ~ metal sub~trate, ~nd new ~nd useful products are provided.

Claims (23)

WE CLAIM:

1. A method of coating a metal substrate with a protective coating which comprises:

(a) providing a substrate metal to be coated, (b) providing an alloy or mixture of at least one metal M1, and at least one other metal M2 selected according to the following criteria:

(1) M1 is susceptible to reaction with a reactive gaseous species of an element X
(X being nitrogen, carbon, boron or silicon) to form a stable compound of Ml and X at a selected temperature and pressure of such reactive species, (2) M2 does not form a stable compound with X under such conditions and it bonds to the substrate on heat treatment of the coated material;

(c) applying such alloy or mixture to a surface of the substrate to provide a coating and (d) effecting selective reaction of M1 with such gaseous species at an elevated temperature under conditions to produce a compound of M1 and X and to avoid or minimize formation of a compound of M2 with X.

2. The method of Claim 1 wherein after step (c) the coating is annealed.

3. The method of Claim 1 wherein the substrate metal is a ferrous alloy.

4. The method of Claim 1 wherein the substrate metal is a non-ferrous alloy.

5. The method of Claim 1 wherein X is nitrogen.

6. The method of Claim 1 wherein X is carbon.

7. The method of Claim 1 wherein X is boron.

8. The method of Claim 1 wherein X is silicon.

9. The method of Claim 1 wherein M1 is selected from the lanthanide metals.

10. The method of Claim 1 wherein M1 is selected from the actinide metals.

11. The method of Claim 1 wherein M2 is selected from the group nickel, cobalt, aluminum, yttrium, chromium and iron.

12. The method of Claim 1 wherein M1 is titanium and M2 is cobalt or nickel.

13. The method of Claim 1 wherein M1 is selected from groups III b, IV b and V b of the Periodic Table.

14. The method of Claim 12 wherein X is nitrogen.

15. The method of Claim 12 wherein X is carbon.

16. A coated metal article comprising:

(a) A metal substrate and (b) a protective coating on and adherent to at least one surface of the metal substrate, such coating comprising an outer layer of a compound M1Xn wherein X is nitrogen, carbon, boron or silicon and n represents the atomic proportion of X to M1 and an inner layer of at least one metal M2 bonded to the substrate, said metals M1 and M2 being selected according to the following criteria:

(1) M1 is susceptible to reaction with a reactive gaseous species of an element X
(x being nitrogen, carbon, boron or silicon) to form h stable compound of M1 and X at a selected temperature and pressure of such reactive species, (2) M2 does not form a stable compound with X under such conditions and it bonds the coating to the substrate.

17. The coated metal article of Claim 16 wherein the metal substrate is a ferrous alloy.

18. The coated metal article of Claim 16 wherein the metal substrate is a non-ferrous alloy.

19. The coated metal article of Claim 16 wherein X is nitrogen.

20. The coated metal article of Claim 16 wherein X is carbon.

21. The coated metal article of Claim 16 wherein X is boron.

22. The coated metal article of Claim 16 wherein X is silicon.

23. The coated metal article of Claim 16 wherein M1 is selected from groups III b, IV b and V b of the Periodic Table.