EP0201531A4

EP0201531A4 - Process for applying hard coatings and the like to metals and resulting product.

Info

Publication number: EP0201531A4
Application number: EP19850905364
Authority: EP
Inventors: Ibrahim Mohamed Allam; David John Rowcliffe
Original assignee: SRI International Inc; Stanford Research Institute
Current assignee: SRI International Inc
Priority date: 1984-10-17
Filing date: 1985-10-16
Publication date: 1988-11-22
Also published as: DK282186A; GB8613492D0; DE3590390T1; EP0201531A1; WO1986002385A1; DK282186D0; GB2176809A; SE8602616L; SE8602616D0; GB2176809B; JPS62500599A

Description

PROCESS FOR APPLYING HARD COATINGS AND THE LIKE TO METALS AND RESULTING PRODUCT

This invention relates to the coating of metals (hereinafter referred to as "substrates" or "substrate metals") with coatings that serve to provide hard surfaces, chemically resistant coatings, etc.

Hard coatings were developed for the purpose of providing a combination of high performance properties such as resistance to friction, wear and corrosion to less expensive metal components. Early techniques used in the application of these coatings were based on surface treatment of metallic substrates by the diffusion of carbon, nitrogen, boron, or silicon, thus generating the hard materials directly in the surface of the substrate. Most of the more recent application techniques involve the deposition of an overlay hard layer as an external coating. Examples of techniques include: Chemical vapor deposition (CVD), physical vapor deposition (PVD), laser fusion, sputtering, flame or plasma spraying, and detonation gun. With the possible exception of CVD processes, these techniques are expensive and limited to the line of sight which may lead to variable thickness and unequal coverage particularly at corners, holes and complex shapes. It is an object of the present invention to provide an improved method of applying to substrate metals coatings of M₁X_n where M₁ is the metal whose compound is to be 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, M₁X_n 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.

In accordance with the present invention, an alloy or a physical mixture of metals is provided comprising two metals M₁ and M₂ which are selected in accordance with the criteria described below. This alloy or metal mixture is then melted to provide 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. The metals M₁ and M₂ are selected according to the following criteria: M₁ forms a thermally stable compound with X (i.e., a nitride, a carbide, a boride or a silicide) when exposed at a high temperature to an atmosphere containing a small concentration of X or of a dissociable molecule or compound of X. The stable compound that M₁ forms with X may be represented as M₁X_n where n represents the atomic ratio of X to M₁.

The metal M₂, under such conditions, does not form a stable compound with X and remains entirely or substantially entirely in metallic form. Further, M₂ is compatible with the substrate metal in the sense that it results in an intermediate layer between the M₁X_n outer layer (resulting from reaction with X) and the substrate, such intermediate layer serving to bond the M₁X_n layer to the substrate. Interdiffusion of M₂ and the substrate metal aids in this bonding effect.

It will be understood that M₁ may be a mixture or alloy of two or more metals meeting the requirements of M₁ 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 is then preferably subjected to an annealing step. The annealing step may be omitted when annealing occurs under conditions of use.

When a 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 the substrate) or by any other suitable process the surface is then exposed to a selectively reactive atmosphere at an appropriate elevated temperature. To form a nitride, carbide, boride or silicide layer on the substrate metal, an appropriate, thermally dissociable compound or molecule of nitrogen, carbon, boron or silicon may be used. Examples of suitable gaseous media are set forth in Table I below including media where X = nitrogen, etc.

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 active 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 precursor species.

There results from this process a structure such as shown in Figure 1 of the drawings. Referring now to Figure 1, this figure 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 M₁X_n 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 layers 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 microns 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 suiting it to its intended use.

Figure 1 is a simplified representation of the coating and substrate. A more accurate representation is shown in Figure 1A in which the substrate 10 and outer layer M₁X_n 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 interdiffusion layer resulting from diffusion of substrate metal outwardly away from the substrate and of M₂ inwardly into the substrate. There is also an intermediate zone I which may be a cermet formed as a composite of M₁X_n and M₂.

The metals M₁ and M₂ will be selected according to the intended use. Table II below lists metals which may be used as M, 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 III; it is required that M₂ be more noble than M₁ in any M₁/M₂ pair. Another factor is the intended use, e.g. whether a hard surface, or a surface which is resistant to aqueous environments is desired, a surface which acts as a lubricant, etc. Also the nature of the substrate should be considered. It will be seen that some metals appear in both tables; that is a metal M₁ appearing in Table II may be used as M₂ (the more noble metal) with a less noble metal M₁ from Table III.

It will be understood that two or more metals chosen from Table II and two or more metals chosen from Table III may be employed to form the coating alloy or mixture. Examples of suitable M₁/M₂ metal pairs including mixtures of two or more metals M₁ and two or more metals M₂ are set forth in Table IV.

It will be understood that not every metal pair will be suitable for all purposes. For example, where M₁ is silicon the coating tends to be brittle; some pairs are better suited for hardness, others for service as thermal barriers, others for oxidation and corrosion resistance, etc.

Examples of eutectic alloys are listed in Table V. It will be understood that not all of these alloys are useful on all substrates. In some cases the melting points are approximate. Numbers indicate the approximate percentage by weight of M₂.

Table VA lists certain tertiary alloys that are useful in the practice of the present invention.

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 N i

76 8 16

77 7 16 79 5 16

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

Superalloys

Cast nickel base such as IN 738

Cast cobalt base such as MAR-M509

Wrought nickel base such as Rene 95

Wrought cobalt base. such as Haynes alloy No. 188

Wrought iron base such as Discaloy

Hastalloy X

RSR 185

Incoloy 901

Coated superalloys (coated for 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

Tool Steels (wrought, cast or powder metallurgy) such as AISIM2; AISIW1

Stainless Steels

Austenitic 304 Ferritic 430 Martensitic 410

Carbon Steels AISI 1018

Alloy Steels

AISI 4140 Maraging 250 Cast irons

Gray, ductile, malleable, alloy UNSF 10009

Non-ferrous Metals

Titanium and titanium alloys, e.g. ASTM Grade 1;

Ti-6A1-4V Nickel and nickel alloys, e.g. nickel 200, Monel 400 Cobalt Copper and its alloys, e.g. C 10100; C 17200;

C 26000; C 95200

Refractory metals and alloys

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.% TiC, balance steel; 10-20% TiC-balance steel

The proportions of M₁ to M₂ may vary widely depending upon such factors as the choice of M₁ and M₂, the nature of the substrate metal, the choice of the reactive gaseous species, the conversion temperature, the purpose of the coating (e.g. whether it is to serve as a thermal barrier or as a hardened surface), etc. The dip coating method is preferred. It is easy to carry out and the molten alloy removes surface oxides (which tend to cause spallation). In this method a molten M₁/M₂ alloy is provided and the substrate alloy is dipped into a body of the coating alloy. The temperature of the alloy and the time during which the substrate is held in the molten alloy will control the thickness and smoothness of the coating. If an aerodynamic surface or a cutting edge is being prepared a smoother surface will be desired than for some other purposes. The thickness of the applied coating can range between a fraction of one micron to a few millimeters. Preferably, a coating of about 300 microns to 400 microns is applied if the purpose is to provide a thermal barrier. A hardened surface need not be as thick. It will be understood that the thickness of the coating will be provided in accordance with the requirements 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 applied to the substrate. Also complex shapes can be coated and the process can be repeated to build up a coating of desired thickness. Typically, the slurry coating technique may be applied as follows: A powdered alloy of M₁ and M₂ is mixed with a mineral spirit and an organic cement such as Nicrobraz 500 (Well Colmonoy Corp.) and MPA-60 (Baker Caster Oil Co.). Typical proportions used in the slurry are coating alloy 45 weight percent, mineral spirit 10 weight percent, and organic cement, 45 weight percent. This mixture is then ground, for example, in a ceramic ball mill using aluminum oxide balls. After separation of the resulting slurry from the alumina balls, it is applied (keeping it stirred to insure uniform dispersion of the particles of alloy in the liquid medium) to the substrate surface and the solvent is evaporated, for example, in air at ambient temperature or at a somewhat elevated temperature. The residue of alloy and cement is then fused onto the surface by heating it to a suitable temperature in an inert atmosphere such as argon that has been passed over hot calcium chips to getter oxygen. The cement will be decomposed and the products of decomposition are volatilized.

If the alloy of M₁ and M₂ has a melting point which is sufficiently high that it exceeds or closely approaches the melting point of the substrate, it may be applied by sputtering, by vapor deposition or some other technique.

It is advantageous to employ M₁ 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.

The following specific examples will serve further to illustrate the practice and advantages of the invention. Example 1.

The substrate metal was tool steel in the form of a rod. The coating alloy was a eutectic alloy containing 71.5% Ti and 28.5% Ni. This eutectic has a melting point of 942°C. The rod was dipped into this alloy at 1000°C for 10 seconds and was removed and annealed for 5 hours at 800°C. It was then exposed to oxygen free nitrogen for 15 hours at 800°C. The nitrogen was passed slowly over the rod at atmospheric pressure. The resulting coating was continuous and adherent. The composition of the titanium nitride, TiN_X, 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 is inexpensive but soft. This provides a way of providing an inexpensive metal with a hard surface.

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 Example 1.

Darker colors obtained at higher temperatures indicated a stoichiometric composition, TiN.

Similar coatings were applied to stainless steel. Example 4.

A eutectic alloy of 83% Zr and 17% Ni (melting point = 961°C) is employed. The substrate metal (tool steel) is dip coated at 1000°C, annealed 3 hours at 1000°C and exposed to nitrogen as in Examples 1 and 3 at 800°C. A uniform adherent coating 2 to 3 microns thick resulted.

Example 5.

A 48% Zr - 52% Cu eutectic alloy, melting point 885ºC was used. Tool steel was dipped into the alloy for 10 seconds at 1000°C and was withdrawn and annealed 5 hours at 1000°C. It was then exposed to nitrogen at one atmosphere for 50 hours at 800°C. A uniform adherent coating resulted.

An advantage of copper as 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 alloy, a eutectic alloy, melting at 875°C was used. Hot dipping was at 1027ºC for 10 seconds; annealing at 900°C for 5 hours; exposure to N₂ at 900°C for 100 hours. An adherent continuous coating resulted. The substrate metal was high speed steel.

Example 7.

Tool steel was coated with a Ti-Ni alloy and annealed as in Example 3. The reactive gas species is methane which may be used with or without an inert gas diluent such as argon or helium. The coated steel rod is exposed to methane at 1000°C for 20 hours. A hard, adherent coating of titanium carbide results.

Example 8.

The procedure of Example 7 may be repeated using BH₃ as the reactive gas species at a temperature above 700ºC, e.g. >700°C to 1000ºC, for ten to twenty hours. A titanium boride coating is formed which is hard and adherent.

Example 9.

The procedure of Example 7 is repeated using silane. Si H₄, as the reactive gas species, with or without a diluting inert gas such as argon or helium. The temperature and time of exposure may be >700°C to 1000°C for ten to twenty hours. A titaniur. silicide coating is formed which is hard and adherer t.

Among other considerations are the following:

The metal M₂ should be compatible with the substrate. For example, it should not form brittle intermetallic compound with metals of the substrate. Preferably it does not alter seriously the mechanical properties of the substrate and has a large range of solid solubility in the substrate. Also it preferably forms a low melting eutectic with M₁. Also it should not form a highly stable carbide, nitride, boride or silicide. For example, if M₁ is to be converted to a nitride, M₂ should not form a stable nitride under the conditions employed to form the M₁ nitride.

In the hot dipping method of application of an M₁/M₂ alloy, uneven surface application may be avoided or diminished by spinning and/or wiping.

The annealing step after application of the alloy or mixture of M₁ and M₂ should be carried out to secure a good bond between the alloy and the substrate.

Conversion of the alloy coating to the final product is preferably carried out by exposure to a slowly flowing stream of the reactive gas at a temperature and pressure sufficient to react the reactive gaseous molecule or compound with M₁ but not such as to react with M₂. It is also advantageous to employ a temperature slightly above the melting point of the coating alloy, e.g. slightly above its eutectic melting point. The presence of a liquid phase promotes migration of M₁ to the surface and displacement of M₂ in the outer layer. If the temperature is below the melting point of the coating alloy and if the compound formed by M₁ and the reactive gaseous species grows fast, M₂ will be entrapped in the growing compound, thus bonding the particles of

M₁X_n. In this case a cermet will be formed which may be advantageous, e.g. a W or Nb carbide cemented by cobalt or nickel.

It will therefore be apparent that a new and useful method of applying M₁X_ncoating to a metal substrate, and new and useful products are provided.

Claims

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 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 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 M, 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 M₁X_n 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 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.

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 M^ is selected from groups III b, IV b and V b of the Periodic Table.