Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C26/00—Coating not provided for in groups C23C2/00 - C23C24/00

Definitions

• 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.
• an alloy or a physical mixture of metals comprising two metals M 1 and M 2 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.
• 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.
• M 1 and M 2 are selected according to the following criteria: M 1 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.
• X i.e., a nitride, a carbide, a boride or a silicide
• the stable compound that M 1 forms with X may be represented as M 1 X n where n represents the atomic ratio of X to M 1 .
• the metal M 2 does not form a stable compound with X and remains entirely or substantially entirely in metallic form. Further, M 2 is compatible with the substrate metal in the sense that it results in an intermediate layer between the M 1 X n outer layer (resulting from reaction with X) and the substrate, such intermediate layer serving to bond the M 1 X n layer to the substrate. Interdiffusion of M 2 and the substrate metal aids in this bonding effect.
• M 1 may be a mixture or alloy of two or more metals meeting the requirements of M 1 and that M 2 may also be a mixture or alloy of two or more metals meeting the requirements of M 2 .
• 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.
• 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/M 2 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.
• a nitride, carbide, boride or silicide layer an appropriate, thermally dissociable compound or molecule of nitrogen, carbon, boron or silicon may be used.
• 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.
• an inert gas e.g. argon
• FIG. 1 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 M 1 X 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 M 1 X 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 M 2 or it may be an interdiffusion layer resulting from diffusion of substrate metal outwardly away from the substrate and of M 2 inwardly into the substrate. There is also an intermediate zone I which may be a cermet formed as a composite of M 1 X n and M 2 .
• D may be an alloy of one or more substrate metals and the metal M 2 or it may be an interdiffusion layer resulting from diffusion of substrate metal outwardly away from the substrate and of M 2 inwardly into the substrate.
• intermediate zone I which may be a cermet formed as a composite of M 1 X n and M 2 .
• the metals M 1 and M 2 will be selected according to the intended use.
• Table II below lists metals which may be used as M
• Table III lists metals that may be used as M 2 .
• Not every metal in Table II may be used with every metal in Table III; it is required that M 2 be more noble than M 1 in any M 1 /M 2 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.
• the nature of the substrate should be considered. It will be seen that some metals appear in both tables; that is a metal M 1 appearing in Table II may be used as M 2 (the more noble metal) with a less noble metal M 1 from Table III.
• eutectic alloys examples 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 M 2 .
• 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 (M 2 ) alloys because they stabilize zirconia in the cubic form.
• M 2 zirconium-noble metal
• Table VI provides examples of metal substrates to which the metal pairs may be applied. Table VI
• Cast nickel base such as IN 738
• Cast cobalt base such as MAR-M509
• Wrought cobalt base such as Haynes alloy No. 188
• Wrought iron base such as Discaloy
• Coated superalloys (coated for corrosion resistance)
• Tool Steels wrought, cast or powder metallurgy
• AISIM2 wrought, cast or powder metallurgy
• AISIW1 AISIW1
• Titanium and titanium alloys e.g. ASTM Grade 1;
• Nickel and nickel alloys e.g. nickel 200, Monel 400 Cobalt Copper and its alloys, e.g. C 10100; C 17200;
• Molybdenum alloys e.g. TZM Niobium alloys, e.g. FS-85 Tantalum alloys, e.g. T-lll Tungsten alloys, e.g. W-Mo alloys
• 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 M 1 to M 2 may vary widely depending upon such factors as the choice of M 1 and M 2 , 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 M 1 /M 2 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.
• 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.
• the slurry coating technique may be applied as follows: A powdered alloy of M 1 and M 2 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.
• the substrate surface 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.
• the alloy of M 1 and M 2 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.
• M 1 and M 2 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 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 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 2 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.
• 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.
• 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 M 2 is that it is a good heat conductor which is helpful in carrying away heat (into the body of the tool) in cutting.
• 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 N 2 at 900°C for 100 hours. An adherent continuous coating resulted.
• the substrate metal was high speed steel.
• 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 7 may be repeated using BH 3 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 7 The procedure of Example 7 is repeated using silane. Si H 4 , 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.
• the metal M 2 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 M 1 . Also it should not form a highly stable carbide, nitride, boride or silicide. For example, if M 1 is to be converted to a nitride, M 2 should not form a stable nitride under the conditions employed to form the M 1 nitride.
• the annealing step after application of the alloy or mixture of M 1 and M 2 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 M 1 but not such as to react with M 2 . 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 M 1 to the surface and displacement of M 2 in the outer layer. If the temperature is below the melting point of the coating alloy and if the compound formed by M 1 and the reactive gaseous species grows fast, M 2 will be entrapped in the growing compound, thus bonding the particles of
• cermet will be formed which may be advantageous, e.g. a W or Nb carbide cemented by cobalt or nickel.

Landscapes

• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Chemical Vapour Deposition (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=24657002

Family Applications (1)

Country Status (7)

Families Citing this family (2)

Family Cites Families (4)

• 1985
  • 1985-10-16 JP JP50470585A patent/JPS62500599A/ja active Pending
  • 1985-10-16 GB GB08613492A patent/GB2176809B/en not_active Expired
  • 1985-10-16 EP EP19850905364 patent/EP0201531A4/fr not_active Withdrawn
  • 1985-10-16 WO PCT/US1985/002031 patent/WO1986002385A1/fr not_active Application Discontinuation
  • 1985-10-16 DE DE19853590390 patent/DE3590390T1/de not_active Withdrawn
• 1986
  • 1986-06-11 SE SE8602616A patent/SE8602616L/ unknown
  • 1986-06-16 DK DK282186A patent/DK282186A/da not_active Application Discontinuation

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events