CA1244304A - Process for applying coatings to metals and resulting product - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

Protective 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 an oxide, a nitride, a carbide, a boride or a silicide. The coating material is a mixture of the metals M1 and M2 of which Ml forms A stable oxide, nitride, carbide, boride or silicide under the prevailing conditions and of which M2 does not form a stable oxide, nitride, carbide, boride or silicide. M2 serves to bond the oxide, etc. of Ml 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

L~304 RPROCESS FOR APPLYING COATINGS
TO METALS AND RESULTING PROD~CT"

This invention relates to the coating of metals (hereinafter referred to as substrates~ or ~substrate metals~) with coatings that 6erve to provide hard surfaces, thermal barriers, oxidation barriers, chemically resistant coatings, etc.

By way of example, certain alloys known as ~superalloys" are used as gas turbine components where high temperature oxidation resistance and high mechanical strength are required. In order to extend the useful tempera~ure 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 $s employed for this purpose because it has a thermal expansion coefficient approximating that of the ~uperalloys and because it functions as an efficient thermal barrier. It has been applied heretofore to alloy substrates by plasma spraying, in which an inner layer or bond coat, for example NiCrAlY alloy, protects 30D~

the superalloy substrate from oxidation and bonds to the superalloy and to the zirconium oxide. 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 i5 expensive and limited to line of sight application.
Variations in coa~ing compositions often occur because of differences in vapor pressures of the coating constituent elements.

It is an object of the present invention to provide an improved method of applying to substrate ¢etals coatings of MlXn where Ml is the metal whose compound is to be applied to the substrate, X is an element such as oxygen, nitroyen, carbon, boron or silicon, and n is a number indicating the atomic proportions of X to M.

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

The above and other objects of the invention will be apparent from the ensuing description and the appended claims.
The inven-tion is illustrated by way of example in the accompanying drawings in which:

~2'~ 30~
-2a-Figure 1 represents a simplified cross-section through a substrate alloy and a coating applied thereto in accordance with the invention;

Figure lA is a more detailed and accurate representation of the cross-section of Figure l; and Figure 2 is a cross-section similar to that of Figure lA showing a coating resulting from application of a cerium-cobalt alloy and ox~dation of the cerium to cerium oxide in accordance with the invention.

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The inventlon 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, Ml constituting not less than 50~ by weight of the alloy or mixture, M2 being present in substantial amount but not exceeding 50%
by weight, M1 and M2 being selected according to the following criteria:
(1) M1 is susceptible to reaction with a reactive ~aseous species of an element x (x being oxygen, 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;

(3) Ml and M2 and the proportions in which they are used being also such that their alloy melts below the melting point of the substrate and at a sufficiently low temperature to avoid substantial degradation of the surface of the substrate;
(c) applying such alloy or mixture to a surface of the substrate (1) by dipping the substrate in a molten alloy of Ml and M2 or (2) by applying a slurry in a volatile liquid of the metals Ml and M2 in finely divided form either as a mixture of the separate metals or as an alloy of Ml and M2, then vaporizing the solvent and fusing the metals;
(d) effecting selective reaction of Ml with such gaseous species at an elevated temperature under conditions to produce a compound of Ml and X and to avoid substanti.al formation of a compound of M2 with X;
(e) said method resulting in a coating which is bonded to the substrate, said coating having an intermediate bonding layer and an outermost layer which is substantially entirely a compound of Ml and X and serves as a protective barrier for the substrate, said ;~24~3V~
3a intermediate bonding layer having (1) an interaction zone and (2) a subscale zone, said interaction zone being composed substantially entirely of M2 bonded to the substrate by the alloying of at least one component of M2 with at least one component of the substrate, and said subscale zone heing composed of an adequate amount of M2 and said compound of M1 so as to establish a firm bond with the interaction zone, said outermost layer and said intermediate bonding layer being formed by said step (d) r said intermediate bonding layer serving to bond said outermost layer to the substrate.
The invention further provides 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 oxygen, 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) M1 is susceptible to reaction with a reactive gaseous species of an element X (X being oxygen, 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 the coating to the substrate.
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 metaI mixture or alloy is reduced to a finely divided state, and the finely divided metal is incorporated in a volatile solvent to form a 3~)4 3b 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: Ml 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 Ml forms with X may be represented as MlXn where n represents the atomic ratio of X to Ml.

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The metal M2, under such conditions, does not form a stable compound with X ~nd remains en~irely or 6ubstantially entirely in ~etallic form. Further, M2 is compatible with the substrate metal in the sense that it results in an intermediate layer between ehe MlXn outer layer (resulting from reaction with X) ~nd the substrate, such intermedi~te l~yer ~erving to bond the MlXn layer to the ~ubstrate. Interdiffusion of M2 and the ~ubstr~te metal 6ids in this bonding effect.

It will be under6tood that Ml may be a mixture or alloy of two or more metal6 meeting the requirements of Ml and that M2 may al~o be a mixture or alloy of two or more metals ~eeting the requirements of M2.

The coating thus formed and ~pplied is then preferably subjected to an annealing ~tep. 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 ~fter the solvent has been ev~porated and the M~M2 metal alloy or mixture is fused onto the 6urface of the substrate) or by ~ny other suitable process the surface is then exposed to ~ selectively reactive atmosphere at an appropriate elevated temperature. Where an oxide coating is desired (i.e. X ~ 0) a mixture of carbon dioxide and carbon monoxide (hereinafter referred to as C02/C0~ may be used. A typical C02/C0 mixture contains 90 percent of C02 and 10 percent of C0. When such a mixture is heated to a high temperature, an equilibrium mixture result~ in accordance with the following equation:

CO + 1/2 2 ~ C2 lZ ~'~3(~4

-5-~he concentration of oxygen in this equilibrium mixture is very small, e.g., et B00~C the equilibrium oxygen partial pressure is approximately 2 x 10 17 atmosphere,~but i5 ~ufficient at 6uch temperature to bring ~bout 6elective oxidation of Ml. Other oxidizing atmospheres may be used, e.g., mixtures of oxygen and inert gases such ~s argon or mixtures of hydrogen and weter v~por which provide oxygen partial pressures lower than the dissociation pressure-~ o the oxides of the metals M2, ~nd bigher than the di~sociation pressure of the oxide ~f Ml.

Where it is desired to form ~ nitride, carbide, boride or ~ilicide layer on the ubstrate metal, an appropri~te, thermally dissociable compound or molecule of nitrogen, carbon, boron or silicon may be u~ed. Examples of suitable gaseous media are set forth in Table I below including media where X - oxygen, nitr4gen, etc.

Table I. Gaseous Media for Forming Oxides, Nitrides, Carbides, Borides and Silicides .
Gaseous Media O H2/H20, CO/CO2, 02/inert gas.

N N2, NH3 or mixtures of the two.

C Methane, acetylene.

B Borane, diborane, borohalides.

Si Silane, trichloro silane, tribromosilane, silicon tetrachloride.

-6-Where a very low partial pressure of the reactive species i6 needed, that species may t~e diluted by fin inert qas, e.g. argon or its concentration may be ~djusted hS in the case of a COfCO2 mixture or ~n H,~H2O mix~ure where the partial pressure of oxygen is adjusted by ~djusting the ratlo of CO ~nd CO2 or H2 ~nd H2O.

There results from this process a ~tructure 6uch as shown in ~igure 1 of the drawings.

Referring now to Figure 1, ~his figure represents a cross-section through a substrate alloy indicated ~t 10 coated with a laminar coating indicated at il. 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 substra~e layer lD
is a~ ~hick 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 ~ubstrate nnd ~hat the layer 13 will have a thickness suiting it to its intended use. If, for example, Dn oxide layer is provided which will act as a thermal barrier, thicker layer may be desired than in the case where the purpose is to provide ~ hard surface.

~Z'~3~4 Figure 1 is a ~implified representation of the coating and substrate. A more accurate representation ~s ~hown in Figure lA in which the substrate 10 and outer layer MlXn are ~s described in Figure 1. However there is ~ diffusion zone D which may be an alloy of one or more substrate metals and the metal M2 or it may be ~n inter-diffusion layer resulting from diffu6ion of sub6trate metal outwardly away from the substrate and f ~2 inwardly into the substrate. There i6 also an intermediate zone I which may be a cermet formed as a composite of MlXn Mnd M2.

The metals Ml and M2 will be selected according to the intended use. Table II below li8t8 metal-s 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 III; 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 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 ~ubstrate should be considered. It will be seen that some metals appear in both tables; that is a metal Ml appearing in Table II may be used as M2 (the ~ore noble metal) with a less noble metal Ml from Table III.

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Table II ~M ) ~ 1--Actinium ~eodymium Aluminum Niobium Barium Prase~dymium Beryllium Samarium Calcium Scandium Cerium Silicon Chromium Tantalum Dysprosium Terbium Erbium Thorium Europium ~hulium Gadolinium Titanium Hafnium Tungsten Holmium Vanadium Lanthanum Ytterbium Lithium Yttrium Magnesium Zirconium Molybdenum Table III (M2) Cobalt Palladium Copper Platinum Gold Rhenium Iridium Rhodium Iron Rubidium Manqanese Ruthenium Molybenum Silver Nickel Tin Osmium Zinc ~ 4'~3~4 It will be understood that ltwo or more metals chosen fro~ Table II and two or ~ore metal~ chosen from Table I~I may be employed to form the coatinq alloy or mixture. Examples of ~uitable Ml/M2 metal p~irs including mixtures of two or more met~ls Ml ~nd ~wo or more metals M2 are set forth in Table IV.

Table IV

Ml M2 Ml M2 _ _ .
Ti Ni Th Ni Ti Fe Th ~e Ti Co Th Co Ti Cu Th Mg Ti Pd Ti + NbNi Ti + ZrCo Th Cu Ti + ZrFe Th Al Ti + ZrCu ~c Al Zr Fe Zr Co Sc Cu Zr Cu Sc Fe Zr . Pd . Zr Pt Sc Pd Z r Rh Sc Ru Zr + Y Ni Y Al Z r + YCo Y Co Zr + Y Fe Zr + Y Pd Y Cu Y Fe 124~3V4 --10~

Table IV (Contld.) Ml M2 Ml - M2 Zr + Nb Ni Zr ~ Bf Ni Y Ni ~f Ni y Pd ~f Cu Y Ru Si Nb si co Si Fe Si Ho Si Ni Si Pd Si Pt Cr Ni Cr Pd Si Ru It will be understood that ~ot every metal pair will be suitable for all purposes. For example, where Ml i6 silicon the coating tends to be brittle; some pairE are better 6uited for hardness, others for service as thermal barriers, others for oxidation and eorrosion resistance, etc.

Example of eutectic alloys are li6ted in ~able V.
It will be understood that not all of these alloys ~re useful on all substrates. In some cases the melting points are approxima~e. Numbers indicate the approxima~e percentage by weight of M2.

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Table V' Eutectic All~ l~e '~
~i -- 28. 5 Ni 942 Ti - 32 Fe 1085 Ti - 28 Co 1025 ~i ~ 50 Cu 955 Ti - 72 Cu 88 5 Ti - 4B Pd lOB0 Zr - 17 Ni 960 Zr - 27 Ni 1010 Zr - 1~ Pe 934 Z~ - 27 Co 1061 Zr - 54 ::u 885 Zr - 27 Pd 1030 Zr - 37 Pt 1185 Zr - 2S Rh 106S
Hf - 72 Ni 1130 Hf - 38 Cu 970 Th - 36 Ni 1037 Th - 17 ~e 875 Th - 30 t:o 975 Th - 22. 5 Cu B80 Th - 75 Al 632 Sc - 45 Al llS0 Sc - 77 ~:u 8~5 Sc - 24 Fe 910 Sc - 22 Pd 1000 Sc - 17 Ru 1100 Y -- 93 Al 640 Y - 19 Al 1100 3~

Table V (Cont'd. ) _lting Point ( C) Y 9. 5 Al 960 Y- 2~ (:o 725 Y- 88 ~u 890 Y- 66 Cu B4 0 Y- 50 Cu 83û
Y27 Cu 7~0 Y - 25 Fe 900 Y - 47 Ni 950 Y -- 25 Ni 802 Y - 34 Pd 903 Y - 28 Pd 907 Y - 17 ~ ~080 Nb -- 76. 5 Ni 1270 Nb -- 48. 4 Ni 1175 Si - 88. 3 Al 577 Si - 37. 8 Co 1259 Si - 84 Cu 802 Si - 42 Fe 120û
Si - 12 Ho 1410 Si - 62 Ni 964 Si - 74 Pd 870 - Si - 77 Pt 979 ~ able VA lists cer~ain tertiary alloys that ~re useful in the practice of the present inventisn.

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Table VA

55.18 Ti - 23O13 Nb - 21.li9 Ni 40.38 Ti 43.S2 ~r 16.10 Ni ~0.07 Ti - 44.35 Zr 15.58 Co 25.37 Ti - 65~69 Zr - 11.94 ~e 17.36 Ti - 38.01 Zr - ~4.63 Cu 69.65 Zr - 16.07 Y - 14.26 Ni 5~.96 Zr - 23.34 Y ~ 20.70 Ni 43.08 2r - 40.98 Y - 15.94 Co 56.76 Zr - 32~43 Y - 10.81 ~e 47.89 Zr - 34,39 Y - 17.72 Pd 56.68 Zr - 22.35 Nb - 20.97 Ni 49~33 Zr - 32.43 ~f - 43.9~ Ni 24.20 Zr - 48.51 Hf - 27.29 Ni Y~trium, calcium ~nd magnesium are especially beneficial in zirconium-noble metal (M2) ~lloys because they stabilize zirconia in the cubic form. Examples of ~uch ternary alloys are as follows.

Zr Y Ca Mg Ni 3~

Table VI provides examples of metal substra~es 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 ~uch as Rene 95 Wrought cobalt base such as Haynes*alloy No. 18B
Wrought iron ~ase ~uch as Discaloy Hastalloy X

Incoloy 901 Coated superalloys (coated for corrosion resistance) Superalloys coated with Co(or Ni) Cr-Al-Y alloy, e.~. 15-25% Cr, 10-15~ Al, 0.5~ Y, balance is Co or Ni Steels Tool Steels (wrought, cast or powder metallurgy) ~uch as AISIM2; AISIWl Stainless Steels Austenitic 304 Ferritic 430 Martensitic 410 Carbon Steels * Trade Mark ~ z~30~

Alloy Steels Maraging 250 Cast irons Gray, ductile, malleable, ~lloy Non-ferrous Metals Titanium ~nd titanium ~lloys, e.g. ASTM Grade l;
~i-6Al-4V
Nickel and nickel alloys, e.g. nickel 200, Monei 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. ~S-85 Tantalum alloys, e.g. T-lll Tungsten alloys, e.g. ~-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 * Trade Mark ~.Z4~3(~

The proportions of Ml to M2 may vary widely depending upon such factors as the choice of Ml and M2, the nature of the ~ubstrate metal, the choice of the reactive gaseous ~pecies, the conversion temperature, th~ purpose of the coat~ng (e.g. whether ~t i6 ~0 6erve as ~ thermal barrier or as a hardened surfa oe ), etc.

~ he dip coating method is preferred. It is easy to carry out and the molten alloy removes surface oxides (which tend ~o cause spallation). In this me~hod a molten Ml/M2 alloy is provided and the substrate ~lloy i8 dipped into ~ body of the coating alloy. The temperature of the alloy snd the time during which the ~ubstrate is held in the molten alloy will control the thickness and 6moothness of the coating. If an aerodynamic surface or a cutting edge is being prepared a smoother ~urface 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 clurry fusion method has the ~dvantage 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 31~

coating technique may be applied as follows: A powdered alloy of Ml ~nd M2 is mixed with ~ mineral pirit and an organic cement 6uch as Nicrobraz 590 (Well Colmonoy Corp.) and MPA-60*(Baker Caster Oil Co.). ~ypical proportions used in tbe filurry sre coating alloy 4~ weight percent, mineral spirit 10 weight percent, ~nd 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 ~lurry 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 ~olvent 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 tempera~ure in an inert atmosphere such as argon that has been passed over hot calcium chips to getter oxygen. ~he cement will be decomposed and the products of decomposition are volatilized.

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

It i6 advantageous to employ Ml and M2 in the form of an alloy which is a eutectic or near eutec~ic mixture. ~his has the advantage that a coating of definite, predictable composi~ion i~ 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.

* Trade Mark - ~2,~3(~

The following ~pecific examples will 6erve further to illustrate the practice ~nd advantages of the invention.

Example 1. The substrate was ~ nic~el base ~uperalloy known ~s IN 738, which has a oomposition ~s follows:

61~ Ni 1.75~ Mo .5% Co 2.6%
16~ Cr 1.75~ Ta 3.4~ Al 0.9% Nb 3 - 44 Ti - - -The coating alloy was $n one case an alloy contalning 90 percent cerium nnd 10 percent cob~lt, nnd in another case an alloy containing 90 percent cerium snd 10 percent nickel. The ~ubstrate was coated by dipping a bar of the ~ubstrate alloy into the ~olten coating alloy. The temperature of the coating alloy was 600C, which is aboYe the liquidus temperatures of the coating alloys. By experiment it was determined that a dipping time of ~bout one minute provided a coating of ~atisfactory thickness.

The bar was then extrac~ed ~rom the melt and was exposed to a CO2/CO mixture containing 90.33 percentage CO2 and 9.67 percent CO. The exposure periods r~nged ~rom 30 minutes to two hours and the temperature o~ exposure was 800~C. The equilibrium oxygen partial pressure of the CO2/CO mixture at 800C is about 2.25 x 10 17 atmosphere, and at 900C it is ~bout 7.19 x 10 15 a~mosphere. The dissociation pressures of CoO were calculated at 800 and .~2443V4 900 to be about 2.75 x 10 16 atmosphere and ~bout 3.59 x 10 4 ~tmosphere, respec~ively, and the ~issociation pressure~ of NiO were calcul4ted to be about 9.97 x 10 15 atmosphere ~nd ~bout 8.98 x 10 13 ~tmosphere, respectively.
Under these circumstances neither cobalt nor niçkel was oxidized.

Each coated BpeCimen was then nnnealed in the absence of oxygen in a horizontal tube furnace ~t 900 or 1000C for periods up to two hour~. This resulted in recrystallizntion of oxide gr~in~ in the intermedlate l~yer.

- Examina-~ion of the treated specimens, treated in this manner with the cerium-cobalt ~lloy, revealed a ~tructure in cross-section as shown in ~igure 2. In Figure 2, as in ~igure 1, the ~hickness of the various layers is not to 6cale, thickness of the layers of the coating being exaggerated.

Referring to ~igure 2, the substrate is shown at 10, an interaction zone ~t 12A, a subscale zone at 12B and a dense oxide zone at 13. The dense oxide zone consists substantially entirely of CeO2; the subscale zone 12R
contains both CeO2 and metallic cobalt and the interaction zone 12A contains cobalt and one or more metals extracted from the substrate.

Similar results are obtained using a cerium-nickel alloy containing 90% cerium and 10% nickel.

3~;)4 Example 2 ~ he co~ting alloy composition was 70~Zr-25~Ni-5~Y
by weight. Yttrium was added to the Zr-Ni coatinq alloy to provide a dopant to 6tabilize ZrO2 in the cubic ~tructure during the selective oxidation stage, ~nd also because there i6 ~ome evidence that yttrium improves the adherence of plasma-sprayed ZrO2 coatings. The weight rati4 of Zr to Ni in this alloy was 2.7, which is ~imilar to that of the NiZr2-NiZr eutectic composition. The 5~Y did not signiflcan~ly ~lter the melting temperature of the Zr-Ni eutectic. The ~ubstrates were dipped into the molten coating alloy at 1027C.

Two ~ubstrate alloys were coated, namely MAR-M509 and Co-lO~Cr-3~Y. The results obtained indicated that the ~rO2-based coatings applied by this technique to Co-Cr-Y
alloy are highly adherent, uniform and have very low porosity~ Little or no diffusion zone was observed between the coating and the ~ubstrate alloy. The coating layer was established totally above the Gubstrate surface, ~nd its composition was not significantly altered by the ~ubstrate constituent EDAX-concentration profiles were determined of different elements within the Zr-ri~h layer after hot dipping the ~ubstrate alloy (Co-lOCr-3Y) in ~he coating alloy, followed by an annealing treatment. The coa~ing layer was about 150-160~ thick with a relatively thin (~ 20~) diffusion zone at the interface with the under-lying Eubstrate. Cr was virtually nonexistent within the coating layer and a small amount of Co diffused from the substrate right through the ~oating to the external surface.

Selective oxidation was conducted ~t 1027C in a gas mixture of hydrogen/water vapor/argon at appropriate proportions to provide ~n oxygen par~.ial pre~sure of about 10 17 ~tm. At thi~ pressure, both ni.ckel ~nd ~obalt ~re thermodynamically fitable in ~he metallic form. The 6cale produced by this process con~is~ of an outer oxide layer ~bout 40~ thick ~nd nn inner 6ub6cale compo~ite layer of about 129~ ~hick. The outer layer coneained only ZrO2 and Y2O3. The subscale al60 ConF~i ted of a ZrO2/Y2O3 matrix, but contained A large number of finely dispersed metallic particles, essentially nickel and cobalt.

Although nickel and cobalt were present uniformly within the outer region of the metallic coating ~fter hot dipping ~nd annealing and before the conversion ~f Zr and Y
into oxides, they were virtually absent from this same region after the elective oxidation treatment. X-ray di~fraction analysis of the surface of the ~ample indicated that this outer oxide layer was formed exclusively of a mixture of monoclinic zirconia and yttria.

It i~ believed that the final distribution of element~ across the duplex coating layer and the ~ubsequent oxide morphology are determined largely by the conditions of the final selective oxidation treatment. We believe that oxidation pr~ceeds as follows: The melt composition at the 6ample surface before the 6elective oxidation treatment consists largely of Zr and Ni, smaller con-centrations of Y and Co, and virtually no Cr. Once oxygen is admitted at PO ~ 10 17 atm, Zr and Y atoms diffuse rapidly in the melt toward the outer oxygen/metal interface to form a ~olid ZrO2/Y2O3 mixture. The more noble elements - lZ4~304 -22~

(Ni ~nd Co) ~re then excluded from the melt and ~ccumulate in the metal ide of the interface. ~he deple~ion of Zr from this mel~ increases the nickel content of the ~lloy and render6 it ~ore refractory. Once the oating alloy ~olidifies, atoms of all elemen~s in ~he zemaining metallic part of the coating become le~s ~obile than in the molten ~tate, snd further oxidation proceed~ a6 ~ ~olid ~tate reaction. The continued growth of the ZrO2~Y2O3 continues to promote a countercurrent ~olid state diffu~ion process in the metal side of the interface in which Zr ~nd Y di~fuse toward the interface, while nickel ~nd cobalt diffufie away from the interface~

The profile indicated that, under ~he external ZrO2/~2O3 l~yer, nickel and cobalt exist as small particle6 embedded in the subscale composite layer. The reason for their existence in such a distribution within a matrix of the ZrO2/Y~O3 subscale is not well under~tood. It should be emphasized that the weight fraction of nickel present in the coating layer, before oxidation, amount~ to about 25~, which correspvnds to about 20~ in volume fraction.
This amount will increase in the ~ubscale efter the exclusion of nickel from the outer ZrO2/Y2O3 external ~cale during selective oxidation. This substantial ~moun~
of nickel, added to cobalt diffusing from ~he ~ubstrate, is expected to remain trapped in the subscale layer of the coating during the completion of ~elective oxidation of Zr and Y.

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The configuration ~nd dist~ibution o nickel and cobal~ within this zone ls likely to be determined by the mechanisms of oxidation of Zr ~nd Y wi~hin the ~ubscale ~one. At least two possibilities exist:

(1) The ~oncentr~tion of nickel ~nd cobal~ in the ~etal ahead of the lnterface becomes very high ~ ~ result of their exclusion from the ~rO2/Y203 ~cale initially formed from the ~elt. Some bac~-diffu ion ~f both elemen~c in the solid state ~s likely to continue during further exposure, bu~ the remaining portion of both elements may b* overrun by the advancing oxide/metal inter}ace. This is believed to be more probable than possibility (23.

~ 2) A transition from internal to external oxidation occurs. After the initial formation of a ZrO2/Y203 layer a~ the surface, ZrO2 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. ~hen, these particles may partially block further Zr-0 reac~ion because the diffusion of oxygen atoms to the reaction front ~of internal oxidation) can occur only in the channels between the particles tha~ were previously precipitated.
Further reaction at the reaction front may occur either by sideways growth o~ the existing particles, which requires a very small supersaturation, or by nucleation of ~ new particle. The sideways growth cf ~he particles can thus lead to a compact oxide layer, which can entrap metallic constituents existing within the ~ame resion.

I_2L~30~

~ n general, regardless of lthe mechanism involved, in determining the morphology ~nd distribution sf the metallic particles within ~he subscale zone, the formation of such a ceramic/metallic composite layer between the outer ceramic layer and the inner metallic substrate i5 highly advantageous. This is due to its ability to reduce the ~tresses generated from the mismatch in coefficients of thermal expansion of the outer ceramic ~oating ~nd the inner metallic substrate.

Coating adhesion was evaluated by expGsure of ~everal test specimens to 10 thermal cycles-between lDO04C
and ambient temperature in air. ~he ZrO2/Y203 coating on the alloy Co-lOCr-3Y remained completely adherent and showed no ~ign of spallation or cracking. Careful metallurgical examination along the whole length of the ~pecimen did not reveal any siqn of eracking. The coating appears completely pore free. Furthermore, microprobe analyses across this section shQwed that the distributions of Zr, Y, Ni, Co, and Cr were e~sentially the ~ame as those samples that had not been cycled. The coatings are not equally effective on all substrates. For example, a similar ZrO2/Y203 coating on the alloy M~R-M509 spalled after the second cycle.

- It is belieYed that the presence of yttrium in both the Co-Cr-Y substrate and in the coating alloy promotes adhesion of the oxide layer.

Another significant observation is as follows:
Zirconia-yttria mixtures have been prepared before but as far as we know no one ha~ heretofore subjected an alloy of zirconium, yttrium and a more noble metal to selective oxidation. Heating the resulting ZrO2-Y203-M2 product at 1100C resulted in the in situ formation of the cubic or the stabilized form of ZrO2.

~Z~3~)~
2~-~xample_3.

The substra~e metal was tool steel in ~he form of a rod. The coating nlloy was a eutectic elloy containing 71.5~ Ti ~nd 28.5~ Ni. Thi~ eutectic bas a melting point of 942C. The rod was dipped into thi~ ~lloy ~t lOOOnC
for 10 ~econds ~nd was removed and annealed for 5 hours at B00C. It was then exposed to oxygen ~ree nitrogen for 15 hours at 800C. The nitroqen was passed ~lowly over the rod at atmospheric pressure. The resulting coa~ing was continuous and adheren~. The composition of the titanium nitride, TiN , depends upon-the temperature and the nitrQgen pressure.

Example 4.

Example 3 was repeated using mild steel as the substrate. A titanium nitride layer was applied.

The coatings of Examples 3 and e are useful because the treated surface is hard. Thi~ is especially helpful with mild steel which i6 inexpensive but soft.
This provides a way of providing an inexpensive metal with a hard surface.

Example 5.

The sa~e procedure was carried out as in Example 3 but at 650~C~ The coating, 2 microns thick, was lighter in color than the coatinq of Example 3.

Darker colors obtained at higher temperatures indicated a 6toichiometric composition, TiN.

Similar coatings were applied to stainless steel.

~ 2 -~6-A eutec~ic ~lloy of B3~ Zr ~nd 17~ Ni ~mel~ing point - 961C) is employed. The substrate ~etal (tool steel) iL dip coated at 1000C, annealed 3 hours st lO00C
and exposed to nitrogen as in Example~ 3 and 5 ~t 800C.
A uni~orm adherent coating 2 to 3 microns thick refiul~ed.

Example 7.

A 4B% Zr - 52~ Cu eutectic alloy, melting point 8B5C was used. Tool cteel was dipped into the ~lloy ~o~
lO ~econds ~t 1000C and was withdrawn and ~nnealed 5 hours at 1000C. It was then exposed to nitrogen at one atmosphere for 50 hours at 80DC. A ~niform adherent ooa~ing resul~ed.

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

~-A 77~ Ti - 23~ Cu alloy, a eutectic alloy, ~elting at 875C was used. Hot dipping was at 1027~C for 10 ~econds; annealing at 900DC for 5 hours; exposure to N2 at 900C for 100 hours. An adherent continuous coating resulted. The ~ubstrate metal was high speed steel.

Example 9.

Tool steel was coated with ~ i alloy and annealed 2S 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 csated ~teel rod is exposed to methane ~t lD00C for 20 hours. A hard, adherent coating of titanium carbide resul~.

-27- ~Z~304 Example 10.

The procedure of Example 9 may be repeated using BH3 as the reaetive gas species ~t a ~empera~ure ~bove 7D0C, e.g. >700C to 1000C, for ten to twenty hour6~ A
titanium boride coating is formed which is hard and adherent~

Example 11.

The procedure of Example 9 i~ repeated u ing silane, Si H4, as ~he reactive gas species, with or without a diluting inert gas such ~B argon or helium.
~he temperature and time of exposure may be >700C to 1000C for ten to twen~y hours. A titanium silicide coating is formed which i~ hard ~nd adherent.

TiO2-M2 coatings may be ~pplied to a substrate metal similarly using an oxygen atmosphere as in Examples 1 and 2. An advantage of TiO2-M2 coatings is th~t TiO2 is ~esi6tant to att~ck by aqueous environments and it al~o inhibits diffusion of bydrogen into the substrate metal.

Among other considerations are the following:

The metal M2 ~hould be compatible with the substrate. Por example, it should not ~orm brittle inter-metallic compound with metals of the substrate. Preferably i~ does not alter 6eriously the mechanical properties of the substrate and has a large range of solid solubility in the ~ubstrate. Al~o it preferably forms a 1GW melting eutectic with Ml. Also it ~hould not form a highly ~table oxide, carbide, nitride, boride or silicide. ~or example~
if Ml is to be conver~ed to ~n oxide, M2 hould no~ form a stable oxide under the conditions employed to form the M
oxide.

4309~
-~8-In the ho~ dipping ~e~hod of application of an Ml/M2 alloy, uneven ~urface ~pplication ~ay be ~voided or diminished by ~pinning and/or wiping.

The annealing step ~ ter application of the alloy or mixture of Ml and M2 6hould be carried out to ~ecure a good bond between the alloy ~nd the 6ubstrate.

Conversion o~ the alloy coating to the final product is preferably carried ou by ~xposure ~o a ~lowly flowing stream of the reactive gas at a temperature ~nd pressure 8uf ficient to react the reactive ga~eous molecule or compound with Ml but not such ~s to react with M2. It ifi also advantageous to employ a temperature ~lightly above the melting point of the coating alloy, e.g. ~lightly above its eutectic melting point~ The presence of a liquid phase promotes migration of Ml 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 Ml and the reactive gaseous species grows fast, M2 will be entrapped in the qrowing compound, tbus bonding the particles of MlXn. In this case a cermet will be formed which may be advantageous, e.g, a W or Nb carbide cemeneed by cobalt or nickel.

It will therefore be apparent th~t a new ~nd useful method of apply~ng MlXn coating to a metal substrate, and new and useful products are provided.

Claims (22)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

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, Ml constituting not less than 50% by weight of the alloy or mixture, M2 being present in substantial amount but not exceeding 50%
by weight, 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 oxygen, 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;
(3) M1 and M2 and the proportions in which they are used being also such that their alloy melts below the melting point of the substrate and at a sufficiently low temperature to avoid substantial degradation of the surface of the substrate;
(c) applying such alloy or mixture to a surface of the substrate (1) by dipping the substrate in a molten alloy of M1 and M2 or (2) by applying a slurry in a volatile liquid of the metals M1 and M2 in finely divided form either as a mixture of the separate metals or as an alloy of M1 and M2, then vaporizing the solvent and fusing the metals;
(d) effecting selective reaction of M1 with such gaseous species at an elevated temperature under conditions to produce a compound of Ml and X and to avoid substantial formation of a compound of M2 with X;
(e) said method resulting in a coating which is bonded to the substrate, said coating having an intermediate bonding layer and an outermost layer which is substantially entirely a compound of M1 and X and serves as a protective barrier for the substrate, said intermediate bonding layer having (1) an interaction zone and (2) a subscale zone, said interaction zone being composed substantially entirely of M2 bonded to the substrate by the alloying of at least one component of M2 with at least one component of the substrate, and said subscale zone being composed of an adequate amount of M2 and said compound of M1 so as to establish a firm bond with the interaction zone, said outermost layer and said intermediate bonding layer being formed by said step (d), said intermediate bonding layer serving to bond said outermost layer to the substrate.

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 the substrate metal is a superalloy.

6. The method of claim 3 wherein the substrate is tool steel.

7. The method of claim 3 wherein the substrate is stainless steel.

8. The method of claim 1 wherein M1 is selected from the lanthanide metals.

9. The method of claim 1 wherein Ml is selected from the actinide metals.

10. The method of claim 1 wherein M1 is cerium.

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 cerium, M2 is cobalt or nickel and the substrate metal is a superalloy.

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

14. 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 oxygen, 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 oxygen, 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.

15. The coated metal article of claim 14 wherein the metal substrate is a ferrous alloy.

16. The coated metal article of claim 14 wherein the metal substrate is a non-ferrous alloy.

17. The coated metal article of claim 14 wherein the metal substrate is stainless steel.

18. The coated metal article of claim 14 wherein the metal substrate is a superalloy.

19. The coated metal article of claim 14 wherein M1 is a lanthanide metal.

20. The coated metal article of claim 14 wherein M1 is an actinide metal.

21. The coated metal of claim 14 wherein M1 is selected from groups III b, IV b and V b of the Periodic Table.

22. The method of claim 1 wherein said alloy is a eutectic composition or is close in its composition to a eutectic composition, such eutectic composition having a melting point substantially below the melting point of the substrate metal and below a temperature which degrades the surface of the substrate metal.