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
  • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
• • 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, thermal barriers, oxidation barriers, chemically resistant coatings, etc.
• certain alloys known as "superalloys” are used as gas turbine components where high temperature oxidation resistance and high mechanical strength are required.
• the alloys In order to extend the useful temperature range, the alloys must be provided with a coating which acts as a thermal barrier to insulate and protect the underlying alloy or substrate from high temperatures and oxidizing conditions to which they are exposed.
• Zirconium oxide is employed for this purpose because it has a thermal expansion coefficient approximating that of the superalloys and because it functions as an efficient thermal barrier.
• an inner layer or bond coat for example NiCrAlY alloy
• an inner layer or bond coat for example NiCrAlY alloy
• the zirconium oxide forms an outer layer or thermal barrier and the -zirconia is partially stabilized with a second oxide such as calcia, yttria or magnesia.
• the plasma spray technique usually results in a nonuniform coating; and it is not applicable or it is difficultly applicable to re-entrant surfaces.
• the plasma sprayed coatings often have microcracks and pinholes that lead to catastrophic failure.
• Thermal barrier coatings can also be applied using electron beam vaporization. This method of application is expensive and limited to line of sight application. Variations in coating compositions often occur because of differences in vapor pressures of the coating constituent elements.
• an alloy or a physical mixture of metals comprising two metals M, and - 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 .
• the alloy may be applied by plasma spraying.
• the metals M, and M ⁇ are selected according to the following criteria: M, forms a thermally stable compound with X (i.e., an oxide, 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 M, forms with X may be represented as M,X_ where n represents the atomic ratio of X to M,.
• the metal M- under such conditions, does not form a stable compound with X and remains entirely or substantially entirely in metallic form.
• M is compatible with the substrate metal in the sense that it results in an intermediate layer between the Mi.X.n. outer layer (resulting from reaction with X) and the substrate, such intermediate layer serving to bond the M,X_ layer to the substrate. Interdiffusion of 2 and the substrate metal aids in this bonding effect.
• M may be a mixture or alloy of two or more metals meeting the requirements of M, and that M. may also be a mixture or alloy of two or more metals meeting the requirements of M ⁇ .
• 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 , 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 mixture of carbon dioxide and carbon monoxide hereinafter referred to as CO./CO
• a typical C0 2 /C0 mixture contains 90 percent of C0 2 and 10 percent of CO.
• -17 pressure is approximately 2 x 10 atmosphere, but is sufficient at such temperature to bring about selective oxidation of M_ .
• Other oxidizing atmospheres may be used, e.g., mixtures of oxygen and inert .gases such as argon or mixtures of hydrogen and water vapor which provide oxygen partial pressures lower than the dissociation pressures of the oxides of the metals M,, and higher than the dissociation pressure of the oxide of M,.
• nitride, carbide, boride or silicide layer an appropriate, thermally dissociable compound or molecule of nitrogen, carbon, boron or silicon may be used.
• suitable gaseous media are set forth in Table I below including media where X * oxygen, nitrogen, etc.
• N N «, NH- or mixtures of the two.
• 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 or its concentration may be adjusted as in the case of a CO/C0 2 mixture or an H 2 H 2 0 mixture where the partial pressure of oxygen is adjusted by adjusting the ratio of CO and CO- or H 2 and H 2 ° ⁇
• 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 Mi,Xn 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. If, for example, an oxide layer is provided which will act as a thermal barrier, a thicker layer may be desired than in the case where the purpose is to provide a hard surface.
• 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,X are as described in Figure 1.
• diffusion zone D which may be an alloy of one or more substrate metals and the metal M 2 or it may be an inter- diffusion 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,X and M 2 .
• the metals M ⁇ and M 2 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 M.». 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,/M 2 pair. Another factor is the intended use, e.g. whether a hard surface, a thermal' barrier, 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 M, appearing in Table II may be used as M 2 (the more noble metal) with a less noble metal M ⁇ from Table III.
• Osmium Zinc 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 M,/M 2 metal pairs including mixtures of two or more metals M 1, and two or more metals 2 are set forth in Table IV.
• Table VA lists certain tertiary alloys that are useful in the practice of the present invention. Table VA
• 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.
• 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, to M 2 may vary widely depending upon such factors as the choice of M. 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).
• a molten M,/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. 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.
• 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
• the alloy of M, 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, 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.
• 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 was a nickel base superalloy known as IN 738, which has a composition as follows:
• the coating alloy was in one case an alloy containing 90 percent cerium and 10 percent cobalt, and in another case an alloy containing 90 percent cerium and 10 percent nickel.
• the substrate was coated by dipping a bar of the substrate alloy into the molten coating alloy.
• the temperature of the coating alloy was 600°C, which is above the liquidus temperatures of the coating alloys. By experiment it was determined that a dipping time of about one minute provided a coating of satisfactory thickness.
• the bar was then extracted from the melt and was exposed to a C0 2 /CO mixture containing 90.33 percentage C0 2 and 9.67 percent CO.
• the exposure periods ranged from 30 minutes to two hours and the temperature of exposure was 800°C.
• CO,/CO mixture at 800 ⁇ C is about 2.25 x 10 atmosphere
• the dissociation pressures of CoO were calculated at 800° and 900° to be about 2.75 x 10 6 atmosphere and about 3.59 x 10 -14 atmosphere, respectively, and the dissociation pressures of NiO were calculated to be about 9.97 x 10 atmosphere and about 8.98 x 10 -13 atmosphere, respectively.
• Each coated specimen was then annealed in the absence of oxygen in a horizontal tube furnace at 900° or 1000°C for periods up to two hours. This resulted in recrystallization of oxide grains in the intermediate layer.
• the substrate is shown at 10, an interaction zone at 12A, a subscale zone at 12B and a dense oxide zone at 13.
• the dense oxide zone consists substantially entirely of Ce0 2 ; the subscale zone 12B contains both e0 2 and metallic cobalt and the interaction zone 12A contains cobalt and one or more metals extracted from the substrate.
• the coating alloy composition was 70%Zr-25%Ni-5%Y by weight. Yttrium was added to the Zr-Ni coating alloy to provide a dopant to stabilize r0 2 in the cubic structure during the selective oxidation stage, and also because there is some evidence that yttrium improves the adherence of plasma-sprayed ZrO_ coatings.
• the weight ratio of Zr to Ni in this alloy was 2.7, which is similar to that of the NiZr 2 -NiZr eutectic composition. The 5%Y did not significantly alter the melting temperature of the Zr-Ni eutectic.
• the substrates were dipped into the molten coating alloy at 1027 ⁇ C.
• EDAX-concentration profiles were determined of different elements within the Zr-rich layer after hot dipping the substrate alloy (Co-10Cr-3Y) in the coating alloy, followed by an annealing treatment.
• Selective oxidation was conducted at 1027°C in a gas mixture of hydrogen/water vapor/argon at appropriate proportions to provide an oxygen partial pressure of about
• the scale produced by this process consists of an outer oxide layer about 40 thick and an inner subscale composite layer of about 120 /U. thick.
• the outer layer contained only Zr0 2 and Y 2 ° *
• the subscale also consisted of a Zr0 2 /Y 2 0. matrix, but contained a large number of finely dispersed metallic particles, essentially nickel and cobalt.
• Zr and Y atoms diffuse ⁇ 2 rapidly in the melt toward the outer oxygen/metal interface to form a solid Zr0 2 /Y 2 0 3 mixture.
• the more noble elements Ni and Co
• the more noble elements are then excluded from the melt and accumulate in the metal side of the interface.
• the depletion of Zr from this melt increases the nickel content of the alloy and renders it more refractory.
• the coating alloy solidifies, atoms of all elements in the remaining metallic part of the coating become less mobile than in the molten state, and further oxidation proceeds as a solid state reaction.
• the continued growth of the Zr0 2 Y 2 0. continues to promote a countercurrent solid state diffusion process in the metal side of the interface in which Zr and Y diffuse toward the interface, while nickel and cobalt diffuse away from the interface.
• Zr0 2 internal oxide particles may form ahead of the interface when the concentration of dissolved oxygen and zirconium exceeds the solubility product necessary for their nucleation. Then, these particles may partially block further Zr-0 reaction because the diffusion of oxygen atoms to the reaction front (of internal oxidation) can occur only in the channels between the particles that were previously precipitated. Further reaction at the reaction front may occur either by sideways growth of the existing particles, which requires a very small supersaturation, or by nucleation of a new particle.
• the sideways growth of the particles can thus lead to a compact oxide layer, which can entrap metallic constituents existing within the same region.
• a ceramic/metallic composite layer between the outer ceramic layer and the inner metallic substrate is highly advantageous. This is due to its ability to reduce the stresses generated from the mismatch in coefficients of thermal expansion of the outer ceramic coating and the inner metallic substrate.
• Coating adhesion was evaluated by exposure of several test specimens to 10 thermal cycles between 1000°C and ambient temperature in air.
• the Zr0 2 /Y-0 3 coating on the alloy Co-10Cr-3Y remained completely adherent and showed no sign of spallation or cracking. Careful metallurgical examination along the whole length of the specimen did not reveal any sign of cracking. The coating, appears completely pore free..
• microprobe analyses across this section showed that the distributions of Zr, Y, Ni, Co, and Cr were essentially the same as those samples that had not been cycled.
• the coatings are not equally effective on all substrates. For example, a similar Zr0 2 Y 2 0 3 coating on the alloy MAR.-M509 spalled after the second cycle.
• 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 depends upon the temperature and the nitrogen pressure.
• Example 3 was repeated using mild steel as the substrate. A titanium nitride layer was applied. *
• the coatings of Examples 3 and 4 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 3 but at 650 ⁇ C.
• the coating 2 microns thick, was lighter in color than the coating of Example 3.
• 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 3 and 5 at 800°C. A uniform adherent coating 2 to 3 microns thick resulted.
• a 48% Zr - 52% Cu eutectic alloy, melting point 885°C 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 1027°C 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 10
• Example 9 may be repeated using BH 3 as the reactive gas species at a temperature above 700 ⁇ C, e.g. >700 ⁇ C to 1000 ⁇ C, for ten to twenty hours.
• a titanium boride coating is formed which is hard and adherent.
• Example 9 The procedure of Example 9 is repeated using silane, Si H., 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 titanium silicide coating is formed which is hard and adherent.
• Ti0 2 -M 2 coatings may be applied to a substrate metal similarly using an oxygen atmosphere as in Examples 1 and 2.
• An advantage of Ti0 2 -M 2 coatings is that Ti0 2 is resistant to attack by aqueous environments and it also inhibits diffusion of hydrogen into the substrate metal.
• the metal M 2 should be compatible with the substrate. For example, it should not form brittle inter- metallic 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,. Also it should not form a highly stable oxide, carbide, nitride, boride or silicide. For example, if M. is to be converted to an oxide, M 2 should not form a stable oxide under the conditions employed to form the M, oxide. In the hot dipping method of application of an M,/M 2 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 M, 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, 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, to the surface and displacement of M 2 in- the outer layer.
• cermet will be formed which may be advantageous, e.g. a W or Nb carbide cemented by cobalt or nickel.

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Citations (16)

• 1985
  • 1985-10-16 DE DE19853590538 patent/DE3590538T1/de not_active Withdrawn
  • 1985-10-16 GB GB08613491A patent/GB2178449B/en not_active Expired
  • 1985-10-16 WO PCT/US1985/002035 patent/WO1986002290A1/en not_active Application Discontinuation
  • 1985-10-16 EP EP19850905955 patent/EP0198078A4/en not_active Withdrawn
  • 1985-10-16 JP JP50520985A patent/JPS62500574A/ja active Pending
• 1986
  • 1986-06-10 SE SE8602596A patent/SE8602596D0/xx unknown
  • 1986-06-16 NO NO1986862389A patent/NO862389D0/no unknown
  • 1986-06-16 DK DK282286A patent/DK282286D0/da not_active Application Discontinuation

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