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
• • 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
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising

Definitions

• This invention relates to the coating of metals, particularly certain alloys, with a protective coating that acts as a thermal or oxidative barrier.
• alloys known as "superalloys” are used as gas turbine components where high temperature oxidation resistance and high mechanical strengths are required. 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.
• Zirconium oxide has heretofore been applied to alloy substrates by plasma spraying.
• the zirconium oxide forms an outer layer or thermal barrier and the zirconium oxide is partially stabilized with a second oxide such as calcium, magnesium or yttrium oxide.
• the plasma spray technique often produces nonuniform coatings and it is not applicable or is difficultly applicable, to re-entrant surfaces.
• the plasma sprayed coatings often have microcracks and pinholes and the adherence between the coating and substrate can be poor. All of these effects can lead to catastrophic failure.
• Thermal barrier coatings can also be applied using sputtering or electron beam vaporization. These methods of application are expensive and limited to line of sight application: Variations in coating compositions often occur during electron beam vaporization because of differences in vapor pressures of the coating constituent elements. Sputtering produces fibrous and segmented struetures which can be penetrated by the corrosive species.
• the metal whose oxide is to provide the thermal barrier forms a stable oxide but the other metal, called M 2 , does not form a stable oxide.
• M 1 the metal whose oxide is to provide the thermal barrier
• M 2 the other metal
• This process often called dip coating because it is advantageously carried out by dipping the articles to be coated into a molten alloy of M 1 and M 2 , is easier to carry out than coatings with a metal oxide by the plasma method and the resulting coat is more adherent and is a better thermal barrier.
• an alloy or a physical mixture of (1) the metal M 2 and (2) zirconium, hafnium or a mixture or alloy of the two metals is provided.
• the second metal is zirconium
• additions of me Is such as yttrium, calcium or magnesium can be made in quantities that are sufficient to stabilize zirconium oxide in the cubic form.
• the metal M 2 is 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 in 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 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 in situ in the slurry method of application.)
• Zirconium and hafnium form thermally stable oxides when exposed to an atmosphere containing a small concentration of oxygen such as that produced by a mixture of carbon dioxide and carbon monoxide at a temperature of about 800°C.
• the metal M 2 under such conditions, does not form a stable oxide and remains entirely or substantially entirely in the form of the unoxidized metal. Further, M 2 is compatible with the substrate metal. It will be understood that M 2 may be a mixture or an alloy of two or more metals meeting the requirements of M 2 .
• Zirconium and hafnium have one or more of the following advantages over cerium and other lanthanide metals:
• the coatings are considerably more adherent to the substrate.
• unoxidized metal from the substrate tends to become incorporated in the oxide layer.
• This metal may then be oxidized when the coated article is exposed to an oxidizing atmosphere. This leads to spallation and ultimate fracture of the coating.
• zirconium is used rather than cerium, this difficulty is not encountered or is encountered in much lesser degree.
• the concentration of oxygen in this equilibrium mixture is very small, e.g., at 827°C the equilibrium oxygen partial pressure is approximately 2 x 10 -14 atmosphere, but is sufficient at such temperature to bring about selective oxidation of zirconium and/or hafnium.
• 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 elements in M 2 , and higher than the dissociation pressure of zirconium oxide and hafnium oxide.
• a mixture of hydrogen, water vapor and an inert gas such as argon is indeed preferred because it will not produce an unwanted carbide.
• Such carbides may result at elevated temperatures, e.g. at 627°C, by reason of the Boudouard reaction:
• the metal M 2 is, depending upon the type of service and the nature of the substrate alloy, preferably selected from Table I. 2
• metals chosen from Table I may be employed to form the M 2 component of the coating alloy or mixture.
• minor amounts of aluminum, yttrium and/or chromium may be present.
• any metal M 2 may be used which does not form a stable oxide at a high temperature in the presence of a very small concentration of oxygen, which serves to bond the zirconium and/or hafnium oxide to the substrate and which is suitable for the intended type of service.
• These also include platinum, palladium, ruthenium or rhodium.
• Proportions of zirconium, hafnium (or mixtures or alloys of both) and M may vary from about 50 to 90% by weight of zirconium and/or hafnium to from about 50 to 10% by weight of M 2 , preferably about 70 to 90% of zirconium and/or hafnium and about 30 to 10% of M 2 .
• the alloy resulting from a mixture of zirconium and/or hafnium with M 2 (plus any minor alloying additions) must have a melting point that is sufficiently low that the properties of the substrate alloy are not degraded by being exposed to the dipping temperature.
• the proportion of zirconium and/or hafnium should be sufficient to form an outer oxide layer sufficient to provide a thermal barrier and to inhibit oxidation of the substrate and the proportion of M 2 should be sufficient to bond the coating to the substrate.
• Table II provides examples of substrate alloys to which the protective coatings are applied in accordance with the present invention. It will be noted that the invention may be applied to superalloys in general and specifically to cobalt and nickel based superalloys.
• the invention may also be applied to any metal substrate which benefits from a coating which is adherent and which provides a thermal barrier and/or protection from oxidation by the ambient atmosphere.
• the metal or metals of the substrate should, of course, be nobler than zirconium or hafnium such that they do not form stable oxides under the conditions of selective oxidation.
• the dip coating method is preferred.
• a molten zirconium and/or hafnium-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 of the coating.
• the thickness of the applied coating can range between 100 micrometers to 1000 micrometers.
• a coating of about 300 micrometers to 400 micrometers is applied. 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.
• the slurry coating technique may be applied as follows: An alloy or a mixture of zirconium and/or hafnium with M 2 is mixed with a mineral spirit and an organic cement such as Nicrobraz 500, (Well Colmonoy Corp.) and MPA-60 (Baker Coaster Oil Co.). Typical proportions used in the slurry are coating metal 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 metal and cement is then fused onto the surface by heating it to a suitable temperature, for example, 1000 °C 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 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 ZrO 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 2 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.
• the scale produced by this process consists of an outer oxide layer about 40 ⁇ m thick and an inner subscale composite layer of about 120 ⁇ m thick.
• the outer layer contained only ZrO 2 and Y 2 O 3 .
• the subscale also consisted of a ZrO 2 /Y 2 O 3 matrix, but contained a large number of finely dispersed metallic particles, essentially nickel and cobalt.
• the weight fraction of nickel present in the coating layer amounts to about 25%, which corresponds to about 20% in volume fraction. This amount will increase in the subscale after the exclusion of nickel from the outer ZrO 2 /Y 2 O 3 external scale during selective oxidation. This substantial amount of nickel, added to cobalt diffusing from the substrate, is expected to remain trapped in the subscale layer of the coating during the completion of selective oxidation of Zr and Y.
• these particles may partially block further Zr-O 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 nucleati ⁇ n 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.
• the formation of such 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 ZrO 2 /Y 2 O 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 ZrO 2 /Y 2 O 3 coating on the alloy MAR-M509 spalled after the second cycle.

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• Chemical & Material Sciences (AREA)
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• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
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• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
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Citations (8)

• 1983
  • 1983-05-13 NL NL8320222A patent/NL8320222A/nl unknown
  • 1983-05-13 GB GB08426443A patent/GB2158844A/en not_active Withdrawn
  • 1983-05-13 EP EP19830902322 patent/EP0140889A4/de active Pending
  • 1983-05-13 DE DE19833390480 patent/DE3390480T1/de not_active Withdrawn
  • 1983-05-13 JP JP58502496A patent/JPS60501162A/ja active Pending
  • 1983-05-13 WO PCT/US1983/000748 patent/WO1984004335A1/en not_active Application Discontinuation
  • 1983-09-12 CA CA000436457A patent/CA1237609A/en not_active Expired
• 1984
  • 1984-04-19 IT IT8448073A patent/IT1209837B/it active
  • 1984-12-18 SE SE8406442A patent/SE8406442L/ not_active Application Discontinuation
  • 1984-12-20 NO NO845144A patent/NO845144L/no unknown

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