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
  • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
  • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
  • C23C10/34—Embedding in a powder mixture, i.e. pack cementation

Definitions

• an accelerating component such as either relatively volatile metals as cadmium, lead, zinc, etc., or certain long chain organic compounds
• an accelerating component such as either relatively volatile metals as cadmium, lead, zinc, etc., or certain long chain organic compounds
• This invention relates to the pack cementation diffusion coating of metals into the surface of metal articles embedded and heated in a diffusion coating pack and, more particularly, to techniques and compositions whereby an accelerator component for the diffusion coating is included in the pack to accelerate or increase the diffusion of the coating metal from the pack into the surface of the articles being coated, permitting either a greater diffusion at a given temperature and/ or satisfactory diffusion coatings at coating temperatures lower than those required with conventional diffusion coating processes.
• diffusion coating techniques to which this invention generally relates will be recognized and understood as those diffusion coating processes in which the metal articles to be coated are embedded in a powder pack including, generally, the coating metal (in this case, such metals as aluminum or antimony), usually an inert filler (such as powdered alumina), and an energizer component (such as a halogen or readily vaporizable halide) for aiding the transfer of the coating metal from the powdered pack to the surface of the articles to be coated, and then heating the articles embedded in such pack in a closed retort (usually in the absence of oxygen) to relatively high temperatures for sufficient time to produce the diffusion of the coating metal into the surface of the articles to the desired depth.
• the coating metal in this case, such metals as aluminum or antimony
• an inert filler such as powdered alumina
• an energizer component such as a halogen or readily vaporizable halide
• Such processes are well known for enhancing the oxidation resistance (particularly for high temperature use), erosion resistance, corrosion resistance, etc., of the base article for a variety of purposes and uses, and, as well understood, such pack cementation diffusion coating techniques may require that the coating step be prolonged for many hours, or even more than a day, at relatively high temperatures (e.g., 18002000 F. and usually higher than 1200 F.), depending upon the particular metal composition of the coating and the thickness of coating desired and other characteristics.
• relatively high temperatures e.g. 18002000 F. and usually higher than 1200 F.
• the tempering temperature for some conventional hardenable stainless steel alloys is approximately 1000 F. If it is attempted to apply any sort of diffusion coating into the surface of an article made from such materials, obviously, the crystallographic structure or mechanical properties of the article will be altered during the coating process if the latter requires heating to a temperature 1000 F. or above, and such alteration of the base metal article may render it mechanically unsuited to the desired use, whether or not a surface diffusion coating is successfully applied thereto.
• certain components of the compressor portions of jet aircraft engines are made of certain high strength steels because they are subjected to tremendous mechanical stresses from centrifugal force, thermal shock, and vibration, although the actual operating temperatures rarely exceed about 900 F., so that there is little need to provide either extremely high temperature materials or high temperature oxidation resistant coatings, as is the case, by contrast, with the turbine components of the jet engine subjected to the much higher temperature of the impinging combustion gases.
• coating materials such as aluminum or antimony are difficult, if not impossible, to diffusion-coat into the surfaces of various ferrous metal alloy articles to achieve a satisfactory coating on the completely fabricated and finished article at coating temperatures less than those which would inherently alter or degrade the mechanical or metallurgical properties of the article being coated.
• coating temperatures less than those which would inherently alter or degrade the mechanical or metallurgical properties of the article being coated.
• adequate coating depths may require extended coating times (up to 30 hours or more) and longer than may be economically desirable from the standpoint of commercial practice.
• alumium diffusion coating pack comprising by weight, 20% aluminum powder, 0.5% ammonium iodide, 0.25% urea, and balance alumina filter, at temperatures below 1000 P.
• a preferred accelerator combination particularly for ferrous base metals may include an addition of 1% of each cadmium and zinc powder to the coating pack. Despite the high vapor pressure achieved with magnesium, magnesium additions appear greatly to inhibit desired aluminum deposition on such ferrous substrates.
• metals such as cadmium provide the desired accelerating effect in the aluminizing of metal articles having high chromium content.
• a conventional cementation pack containing about 20% aluminum powder 0.5 ammonium iodide in a preponderant amount of alumina filler (with or without 0.25% urea), a substantial and satisfactory amount of aluminum was deposited in accordance herewithin into a chromium substrate at temperatures as low as only 900 F.
• a coating of 1-2 mil thick was readily and satisfactorily deposited in a 30 hour coating cycle at 900 F. utilizing a cadmium accelerator in accordance herewith, whereas a coating of only less than 0.1 mil thick was deposited on the same chromium substrate under the same coating conditions but without the addition of the cadmium accelerator.
• AMS 5616 steel AMS 6304 steels (generally characterized as including 1% Cr, 0.55% M0, 0.3% V, and with carbon contents up to around 0.5%) and 174 PH stainless steels (characterized as being precipitation hardenable steels containing about 17% Cr, 4% Ni, 3% Cu, and smaller amounts of Co, Mn, and Si), where attempts to produce satisfactory coatings at comparable temperature and time cycles may fail without the accelerating addition of lead powder to the coating pack.
• lead may also be definitely preferred to cadmium for certain applications where cadmium is soluble in the substrate (e.g., nickel substrates) being coated, just as cadmium may be preferred to zinc for use with ferrous substrates in which zinc is relatively soluble.
• the effect or degree of acceleration in accordance herewith does not appear to vary appreciably with the selection of the particular halogen or vaporizable halide used as the energizer in the coating pack. That is, cadmium accelerates the aluminizing of steels at 900 F. to a substantial and satisfactory extent in accordance herewith whether the energizer used be an iodide, bromide, chloride, or fluoride. The thickest coatings are achieved with the iodide, but this is not believed to be a function of the accelerator addition to the pack.
• the accelerators in accordance herewith achieve substantially the same degree of acceleration of coating deposition with metal articles which have been previously aluminized according to this or other techniques as are achieved with uncoated or virgin articles, thus suggesting that the accelerating effect is not a surface-related phenomenon.
• Cadmium does not appear notably to enhance the deposition of chromium or titanium coating metals on steel articles below 1000 F., but the reason appears to be the low solubility of chromium or titanium in iron at such relatively low temperatures. Acceleration of the coating of such materials at higher temperatures, however, is also to be comprehended within this disclosure.
• Both cadmium and lead when either was present as a 1% addition in a pack comprising 20% antimony, 0.5% ammonium iodide, and balance alumina, increased the rate of antimony deposition in a variety of substrates, with a particularly high degree of acceleration when using steels, such as AMS 6304, and cobalt substrates and, to a smaller degree, nickel substrates.
• Both cadmium and lead produce satisfactory results as accelerating additives in accordance herewith in the aluminizing of the metal articles formed of titanium and titanium alloys (such as those also containing about 6% aluminum and 4% vanadium).
• a 1% cadmium accelerator addition doubled the amount of aluminum deposition on titanium during a 12-hour cycle, while an addition of 1% lead increased the deposition by about 1.5 times, and microprobe analyses indicated that neither lead nor cadmium was dissolved or included in the coating or the metal article being coated.
• Microprobe analyses of the various coatings produced in accordance herewith have indicated that aluminum of 1 mil coatings produced on AMS 6304 steel from a pack comprising 20% aluminum, 1% each cadmium and zinc, 0.5% ammonium iodide, and 0.25% urea,- with the balance being tabular alumina, by heating at 900 F. for 30 hours showed about 5-10% zinc and about 60-65% aluminum at the substrate-coating interface, the balance being iron.
• Lead functions satisfactorily as an accelerating addition in accordance herewith in aluminizing treatments at 900 F. with any of the substrate materials iron, nickel, or cobalt.
• lead is a less active or effective accelerator than cadmium with certain materials
• the addition of 1% lead to a conventional coating pack as noted above more than trebled the aluminum deposition (for the same temperature and time) with iron, nickel,
• Both cadmium and lead have relatively high vapor pressures at 900 F., and the iodides thereof have similar thermodynamic stabilities at this temperature and also high vapor pressures. These factors are believed to be of significance in attempting to explain the mechanics of the accelerating results in accordance herewith. It may be noted, for example, that zinc oxide will reduce aluminum tri-iodide to zinc iodide and alumina at relatively high temperatures (perhaps 230 C.) after prolonged reaction. Similarly, cadmium oxide will reduce aluminum tri-iodide to cadmium di-iodide and alumina under substantially the same conditions.
• the acceleration in accordance herewith occurs by an increase in the reduction of the aluminum tri-iodide (formed in the pack by reaction of the aluminum powder and iodide energizer) with cadmium and/or zincthus hav ing the accelerator additives effecting a marked increase in the kinetics of the aluminum tri-iodide reduction necessary for satisfactory deposition for diffusion of aluminum.
• X-ray analysis indicates that the intermetallic compound FeAl is formed in the surface coating on the ferrous article being coated at 900 F. with cadmi-um acceleration in accordance herewith, which compound can be converted to Fe Al by further diffusion treatment at higher temperatures of 1100 F. and above.
• cadmi-um acceleration in accordance herewith, which compound can be converted to Fe Al by further diffusion treatment at higher temperatures of 1100 F. and above.
• attempts to produce coatings on steel at lower temperatures and comprising FeAl have generally failed, with principally the compound Fe Al being formed and then only at higher coating temperatures, perhaps indicating that one effect of the accelerating technique in accordance herewith is to provide for more rapid deposition at lower temperatures where solid state diffusion of the coating material into the article being coated is relatively slow.
• the coating thicknesses on the alloys were less than produced under similar conditions on pure nickel or pure cobalt, with the thinner coatings being apparently attributable to the slower rates of solid state diffusion of aluminum in the superalloys compared to pure nickel and cobalt. Nevertheless, obtaining of any significant coatings on such materials at temperatures as low as 1000 F. (and they are usually coated at above 1500 F.) is a clear indication of the accelerating effects on conventional aluminizing techniques produced by the accelerator additions in accordance herewith.
• the amount of aluminum deposited increases with the aluminum content of the pack, and is affected by other factors well known in this art, but, as noted above, the quantity of depositions appears substantially not to be a function of the accelerator concentration provided the latter is at least about 0.25% of the pack.
• the quantity of depositions appears substantially not to be a function of the accelerator concentration provided the latter is at least about 0.25% of the pack.
• packs containing no more than about 5% aluminum produced rather thin coatings, whereas packs containing over 50% aluminum produced thick coatings that exhibited excessively rough surfaces.
• a range of about 20% to 30% by weight of aluminum in the pack is generally preferred from the practical standpoint of coating quality, although acceleration of aluminum deposition in accordance herewith is achieved at virtually any practical concentration of aluminum metal in the coating pack.
• a preferred concentration from a practical point of view for the accelerator is about 1% by weight of the pack.
• halogen energizer appears not to be controlling as to the accelerator techniques hereof.
• the energizer concentration (considered as ammonium halide) should be maintained at or above about 0.25% by weight of the pack.
• the amount of aluminum deposited from a 20% aluminum pack containing 1% cadmium accelerator may increase as much as five-fold if the ammonium iodide content is increased from 0.25% to 0.5 although the amount of aluminum deposited even at the 0.25% level was still greater than that achieved in comparable packs without the accelerator additives.
• the results discussed heretofore have involved utilizing one or more of the metals calcium, lead, tin, or zinc as the accelerating additive, primarily for purposes of illustration, in enhancing the deposition of coating materials such as aluminum or antimony, at relatively low temperatures (no more than 1000 F.) in the diffusion coating of base meltal articles or alloys composed primarily of iron, chromium, nickel, cobalt, titanium, etc., and using coating packs which, prior to the accelerating addition, are generally conventional in utilizing any one of the four halogens, or halides thereof, as diffusion energizers and alumina as an illustrative inert filler-all in known and well understood manner.
• organic accelerators may be incorporated in the powdered packs in a variety of ways, satisfactory results have been achieved in accordance herewith by admixing (for example, in a ball mill) into the pack powder a solution of the organic accelerator dissolved in an appropriate solvent (such as ethanol) and then removing excess solvent by evaporation, drying, or vacuum, prior to utilizing the pack.
• an appropriate solvent such as ethanol
• organic materials having a reactive hydroxy group to liberate a reducing hydrogen ion
• a large enough molecule in addition to the hydroxy radical to have a relatively high decomposition temperature generally within the range of the coating temperatures desired
• a low volatility produce satisfactory results in accordance herewith.
• the organic accelerator is thermally decomposed during the coating operation to liberate reducing hydrogen in situ in the coating pack enclosed in the coating retort.
• This reducing hydrogen effectively reduces the halide of the coating metal (for example, aluminum tri-iodide) to present the coating metal to the substrate in vapor phase or other reactive condition for diffusion into the surface of the article being coated-substantially in the manner discussed above for the reduction of aluminum tri-iodide by the zinc or cadmium or other metallic accelerator additives in accordance herewith, thus to increase or concentrate or accelerate the kinetics of presentation of diffusable coating material to the surface of the article being coated at the desired low coating temperatures and/or within the desired shortened coating reaction times.
• the coating metal for example, aluminum tri-iodide
• organic materials in addition to those specifically noted above, are readily selected to function thermodynamically to achieve the coating acceleration results in accordance herewith. Indeed, although many of the physical and chemical properties of organic materials are generally thought to be inimical to the utilization of such materials as active ingredients in high temperature metal lurgical techniques such as those to which this invention relates, the organic accelerators may -be preferred in many applications of this invention even to the metallic accelerators mentioned above since, even in vapor phase, the organic materials may be selected to be less toxic and noxious than, for example, cadmium or lead vapors, especially with large scale commercial mass production installations.
• Such enhanced results are achieved merely by the inclusion or addition in a conventional powdered coating pack of an accelerator additive and, particularly, one which, whether metal or organic, decomposes or reacts during the coating step to accelerate decomposition of the halide of coating material conventionally formed for the transition 12 thereof as a diffusion coating into the substrate being coated and/or otherwise enhance the thermodynamics or kinetics of the coating reaction.
• an accelerator additive particularly, one which, whether metal or organic, decomposes or reacts during the coating step to accelerate decomposition of the halide of coating material conventionally formed for the transition 12 thereof as a diffusion coating into the substrate being coated and/or otherwise enhance the thermodynamics or kinetics of the coating reaction.

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• 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)
• Laminated Bodies (AREA)
• Physical Vapour Deposition (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

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Cited By (7)

• 1968
  • 1968-05-31 US US733287A patent/US3589935A/en not_active Expired - Lifetime
• 1969
  • 1969-04-14 GB GB09021/69A patent/GB1243459A/en not_active Expired
  • 1969-05-19 SE SE07043/69A patent/SE362269B/xx unknown
  • 1969-05-19 FR FR6916052A patent/FR2010495A1/fr not_active Withdrawn
  • 1969-05-20 DE DE1925482A patent/DE1925482C3/de not_active Expired
  • 1969-05-20 JP JP44038692A patent/JPS4934889B1/ja active Pending

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