DE2614613A1 - PROCEDURE FOR PULLING OBJECTS - Google Patents

Description

The Secretary of State for Defense in Her Britannic Majesty's Government of the United Kingdom of Great Britain

and Northern Ireland,

London

(Great Britain)

Process for coating objects

The invention relates to methods of coating of objects with diffusion coatings and in particular, although not exclusively, on coatings for gas turbine engine parts, z. B. turbine blades, in order to increase their high temperature corrosion resistance.

It is known to apply metal-like or metalized coatings by means of halide-activated solid insert cementation to generate at constant pressure. Alitized and chrome-plated coatings were already applied using this method Nickel, cobalt and iron based underlays applied.

As a well-known solid cementation process such as US Pat. No. 3,257,230 describes a process for coating cobalt or nickel-based alloys, after which an insert bed with a metallic coating material, such as. B. aluminum, chromium, iron or silicon, one

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a source of halide, such as. B. ammonium halide, containing carrier and a moderator metal around the object to be coated her-m and ^{heated to 760 to 1204 0} C for 15 minutes to 40 hours. During this heating, the aluminum or other coating material is transferred to and deposited on the surface of the article.

In such a process, the halide vapor flows between the coating material and the article mainly by diffusion and, as a result, shrinkage such a process occurs fairly slowly and has a relatively low "feed force", i.e. H. the advance of halide vapors diffusion takes place effectively only over very short distances. If you have objects on their exterior Coated surfaces, this low "feed force" is usually not essential because of the embedding of the object to be coated in a particle bed of the coating material, the element to be transferred is very much close to the object.

In contrast to this is the coating of the inner surfaces such parts as e.g. B. bores, after this particle bed cementation process considerably more difficult to achieve. This applies in particular to the metallization of the surfaces pure bores and cavities, as you can z. B. applies to gas turbine engine parts. U.S. Patents 3,079,276 and GB-PS 1 315 228 describe the filling of holes and cavities in gas turbine engine components with a PuI-Verbettverbindungen, however, such procedures are known to be both difficult and time consuming and not always bring the desired effect, even if it is relatively wide and simple. Drilling is.

It is also known to have coatings of metals and compounds to be applied to internal surfaces by

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the heated surfaces a thermally decomposable metal plating gas, such as B. nickel carbonyl, exposing. Such a process is described in GB-PS 1,070,396.

The invention is based on the object of an improved method for producing diffusion coatings on a Specify object in the form of metal-like or metallized layers, which is particularly suitable for covering fine bores or narrow cavities in gas turbine engine parts and other items.

The invention, with which this object is achieved, is a method for covering an object with a Diffusion coating, after which the article is surrounded in a chamber with a bed of particles used as the coating material Aluminum, chromium, titanium, zirconium, tantalum, niobium, yttrium, rare earth metals, boron or silicon elementary or as chemical Compound and also contains a halide activator, and the contents of the chamber on one for transferring the coating material on the surface of the article and forming the diffusion coating thereon at a sufficient temperature is, with the indication that the pressure of an inert gas or reducing gas or mixture contained in the chamber of these gases is changed cyclically.

The process according to the invention is preferably carried out at a pressure which is substantially below atmospheric pressure, i. H. below about 100 torr, and at as high a cycle frequency as would be required with the transport of a sufficient amount of the Gas through the particle feed bed is compatible per cycle. The ratio of the upper pressure limit to the lower pressure limit is preferably as high as practicable and compatible with the cycle frequency. Suitable pressure ranges are about 50 torr to about 10 torr, preferably at cycle frequencies of at least two cycles per minute. General are

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higher frequencies are beneficial for increasing the ratio of those applied on the inside to those applied on the outside Coating thickness.

The halide activator is preferably selected from a group of inorganic halides of low volatility at the coating temperature chosen so that the halide loss from the chamber used is low. Preferably, a sufficient amount of halide activator is used to ensure that at the end of the process some halide activator is still present in the feed bed. Examples of halide activators low volatility are given in Table 1.

Table 1

Equilibrium vapor, sublimation, or dissociation pressures of some low volatility halide activators

material Temp. "C pressure AlF ₃ 927 1.3 NaF 927 0.1 NaCl 927 2.4 NaBr 927 4.8 NaI 927 15.0 Theatrical Version 927 1.2 KC1 927 4.0 KBr 927 6.5 AI 927 12.3

material Temp. ^U C pressure LiI 827 47 CrF ₃ 785 approx. 0.01 CrF ₂ 927 approx. 0.001 CrCl ₂ 927 5.0 CrBr ₂ 810 0.9 CrI ₂ 793 1.4 CoF ₂ 927 0.05 FeF- 927 0.02

Halides of higher volatility are given in Table 2.

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Table 2

Equilibrium vapor, sublimation or dissociation pressures some high volatility halide activators

material Temp. ⁰ C pressure material Temp. ⁰ C pressure Ci ₂ - 34 760 NH ₄ Br 397 760 ^Br 2 61 760 ¹ I. 183 760 AlCl ₃ 180 760 HCl - 167 760 AlBr ₃ 225 760 HBr - 35 760 AlI ₃ 385 760 HI 100 760 PeCl ₃ 319 760 NH ₄ F - 760 FeCl ₂ 934 760 NH ₄ Cl 397 760 FeBr ₂ 927 570

The choice of halide activator must also take into account factors other than volatility. In particular, the activator must also be suitable for achieving the necessary chemical equilibria, which lead to the transport of gas there and back and to suitable interaction with the coating material and the object to be coated at the coating temperature which should not exceed the temperature at which there is a noticeable deterioration in the Properties of the object to be coated occurs. In the case of certain Nikkei and cobalt-based alloys, for example, a marked deterioration of the properties at the coating temperatures may occur above about 900 ⁰ C. Other halides which can be used in the process of the invention are double halides, e.g. B. Sodium cryolite, Na ^ AlF _fi . It may also be desirable to use a mixture of halide activators, e.g. B. NaF / NaCl / NaBr or KF / NaF / LiF to use to increase the efficiency of the deposition process.

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In the production of aluminizing coatings according to the inventive method, aluminum fluoride, A1P_, was used as found to be particularly effective because it gives alitized coatings of good quality as well as satisfactory uniformity and distribution achieved on both inner and outer surfaces of nickel-based gas turbine blades. Preferably the relative proportions of the aluminum starting material (coating material) and the aluminum trifluoride are such that aluminum trifluoride crystals are not formed.

In one embodiment of the method according to the invention keeping the article to be coated out of direct contact with the particle insert bed by keeping it inside a cage which in turn is embedded in the particulate feed. A preferred setup for one such cage is such that it allows vapors to readily flow through from the particle feed bed into the interior of the cage, however, prevents or retards the outflow to the outside of the cage. A cage according to the invention has sides and one Upper side made of imperforate material, e.g. B. nickel sheet or plates, while the cage base made of wire mesh or consists of a network, whereby steam can easily flow through. This training has the effect of increasing the length of the path of the inert and / or reducing gas through the particle feed bed, thereby a more efficient delivery of the active Halide vapor is promoted.

The particle feed bed may be a carrier, e.g. B. of refractory oxide, for receiving the coating material or for Contain dilution of the insert bed. A refractory oxide support can be a liquid aluminum coating material take up.

The articles coated according to the invention can be made from

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any material that can be solidified by cementation can be coated. Materials usually coated by such cementation are nickel, cobalt and Iron-based alloys and the refractory metals of groups IV, V and VI of the periodic table. In addition to these Materials can advantageously also carbon and carbon-containing materials, e.g. B. tungsten carbide this procedure can be overdone. In particular, titanium carbide coatings can be applied to a cemented carbide article produce.

An example of the method according to the invention applied to aluminizing will now be given with reference to the single figure of FIG Drawing will be explained in more detail. This figure shows a leak-tight chamber and an auxiliary device in or with which the Procedure is feasible.

It can be seen in the drawing that the chamber contains a stove pipe 9 made of MuI-lit, which is surrounded at its lower end by an aluminum oxide pipe 5, electrically heated by a heating coil 4, which is in a thermally insulated box 6, on the top of which is a nickel foil heat shield 7 is attached. A gas turbine blade 1 made of a nickel-based alloy, which is to be provided with a coating, is located in an insert bed 2, which consists of a powdery mixture of aluminum, AlF- contained in the furnace tube 9 and covered by a perforated cover 3. and Al ₂ O ₃ .

The furnace pipe 9 is connected to an auxiliary device by means of a pipe 20 for continuous and cyclical changing of the pressure in: pipe 9 connected. The auxiliary device includes an argon source 26 and a vacuum pump 27, which are connected to the pipe 20 by a time-controlled valve 24 or 25 and each one Needle valve 22 and 23 are connected. A (not shown)

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Mercury anometer is with a branch 21 of the pipe 20 connected and is used to measure the pressure fluctuations in the furnace pipe 9.

The upper end of the stovepipe 9 is through an end plate 19 closed, which is bolted to the stove pipe with a flange 10 Is. A pair of O-ring seals 13, 14 make a gas-tight one Sealing between the end plate 19 and the stovepipe. A screw cap 28 is screwed onto a thread on a cylindrical part of the end plate 19 and has an O-ring seal 12, which provide a gas-tight seal between the end plate and a stainless steel tube 17 extending through the screw cap 28.

The upper part of the stovepipe 9 is water-cooled, the water passing through a copper pipe 18 to the upper end of the stovepipe Incoming- The stainless steel pipe 17 surrounding the pipe receives the water backflow. A netting 11 in good thermal contact with the end of the cooling tube 17 extends across the end of a Hickel tube 8, which is concentric in the upper part of the furnace tube 9 above the insulated box 6 attached, open at the bottom and except for two small ones Holes in its closed upper end are blocked off from the Kamner gases. The temperature of the deployment bed is detected by a (not shown) ^ hermoelement.

The method according to the invention is illustrated by the following examples explained in more detail:

example 1

A Gastarbinenschaiifelabschnltt of ^m PER 100 "alloy was aan with a hole of about 1.5 and a ram diameter of about 110 Uinge after erfindungsgemäSen methods, the method mafaBte alitlert in a chamber. Embedding the

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Shovel section in a powder mixture of 14 g AlF ₃ , 14 g Al and 388 g Al ₃ O ₃ , increasing the temperature of the chamber and its contents to 9 00 ⁰ C and setting the time-controlled valves to maintain an argon flow into the chamber for 3 Seconds while the pressure was increased from 6 torr to 28 torr, a constant pressure of about 28 torr for 20 seconds, and a suction period of 7 seconds while the pressure was decreased to about 6 torr. After 10 hours at the same temperature, the chamber was cooled and the blade section removed. The investigation showed that the surface of the hole was evenly coated with an alitized layer with an average thickness of about 35 μm. The thickness distribution of the coating over the length of the hole can be read from the following figures:

Distance from one end of the hole (mm) 10 20 30 40 50 Coating thickness (μπι) 40 40 35 30 30

Example 2

In another example, a turbine blade from "IN 100" was alloy with holes of a diameter of mm of about 1.5 and a length of about 70 mm for five hours at 900 ⁰ C in a nickel mesh cage alitized, in turn, in a pulverulent starting bed mixture of 6 , 5 g AlF ₃ , 10.6 g Al and 330 g Al-Oo was embedded. The argon pressure range varied from 14 to 58 torr and the pressure cycle frequency was 6 cycles per minute. Shiny, metallic-looking and particularly smooth textured alitized layers were created on both the inner and outer surfaces of the blade. The layer thickness inside the hole was measured near the top, halfway along and near the bottom as 12, 8 or 12 μm. The thickness measured along the outer surface at corresponding points was 25 or 30 μm.

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Example 3

In another example, a turbine blade has been "Nimonic 105" alloy with holes of two different diameters, namely about 0.8 and 1.5 mm, but the same length of about 60 mm alitized for six hours at 900 ⁰ C in a nickel mesh cage, which in turn was embedded in a powder bed mixture of 6.6 g AlF ₃ , 10.5 g Al and 330 g AljO ₃ . The argon pressure range was from 4 to 44 torr and the pressure cycle frequency was 6 cycles per minute. Alitized layers of equivalent quality to that obtained in the previous example were produced on both the inner and outer surfaces of the blade. The inner surface of the hole with the larger cross-section had coating thicknesses of 30, 20 and 30 μm in the upper, middle and lower areas. The corresponding thicknesses for the hole with a smaller cross section were 15 or 12 or 15 μm. The coating thicknesses on the outer surface at approximately the same points were 65, 60 and 60 μm.

Example 4

In yet another example of aluminizing using AlF ₃ , a turbine blade was made from "MARM 246" alloy with holes of two different cross-sections, namely one with a major and minor axis of about 2 and 0.5 mm and the other with a circular diameter of about 2 mm, but in both cases with the same length of about 60 mm, ^{alitized for six hours at 900 0} C in a nickel mesh cage, which in turn is in a powder mixture of 6.6 g AlF ₃ , 10.6 g Al and 330 g Al ^ O _{3 was} embedded. The argon pressure range was from 12 to 52 torr and the pressure cycle frequency was 6 cycles per minute. There were aluminized layers of equivalent quality on both the inner and the outer

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Surfaces of the blade generated. The inner layer thickness the length of the hole with an approximately elliptical cross-section was near the top, halfway up, and Dntereimde 20 or 15 or 25 paa. The corresponding values for the! Hole »it circular cross-sections were 40, 40 and 45 µm, respectively. The corresponding values for the outer surface were 60 and 65 or 65 | im.

Example 5

In one example of Alitierens with SaCl as a halide, a turbine blade from "IN 100" was alloy alitized for five hours at 900 ⁰ C in a nickel mesh cage, which, in turn, g in a Pulvereinsatzmiscfaung 20 MACL, 14 g of Al and 300 g of Al ₂ O ₃ was embedded. The argon pressure range was from 8 to 42 torr and the pressure cycle frequency was 6 cycles per minute. A 2 µm thick layer was formed inside a hole 1.8 mm in diameter and 40 mm in length. The thickness of the outer coating layer was about 16 µm.

Example 6

In one example of Alitierens with HaF as a halide, a portion of "IH 100" alloy was alitized for five hours at 900 ⁰ C in a nickel mesh cage, which, in turn, g in a powder feed mixture of 14.7 NaF, 13.6 g of Al and 330 g Al ₂ O _{3 was} embedded. The argon pressure range was from 12 to 56 torr and the pressure cycle frequency was 6 cycles per minute. An aluminizing layer with a thickness of 8 μm was produced in a hole with a diameter of about 1 mm and a length of about 40 mm. The aluminizing layer thickness on the outer surface was about 30 μm.

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The method according to the invention can also be used for coating the inner and outer surfaces of articles with other types of diffusion coatings that have hitherto been applied applied according to known cementation methods were, e.g. B. in addition to aluminizing also coatings by diffusion of chromium, titanium, tantalum, boron and silicon. It will be understood by those skilled in the art that some activators are better suited than others for a certain metallization process and that therefore a certain care in the choice the activators must be observed. Low volatility activators, which are not readily available can be synthesized inside or outside the coating chamber and put into the insert bed can be introduced. According to one embodiment of the invention, for example, SiCl ^ made of silicon and Synthesize ammonium chloride for use in diffusing silicon.

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Claims (18)

26H613 Claims * _ \) Process for coating an object with a diffusion coating, after which the counterstapd is surrounded in a chamber with a particle insert bed, the coating material aluminum, chromium, titanium, zirconium, tantalum, niobium, yttrium, rare earth metals, boron or silicon elemental or contains as a chemical compound and also a halide activator, and the contents of the chamber at a temperature sufficient to transfer the coating material to the surface of the object and form the diffusiom coating thereon, characterized in that the pressure of an inert gas or reducing gas contained in the chamber or a mixture of these gases is changed cyclically. 2. The method according to claim 1, characterized in that dae the maximum pressure of the gas or gases is 100 Torr. 3. The method according to claim 1, characterized in that da8 the maximum pressure of the gas or gases 50 Torr and the minimum pressure 10 Torr. 4. The method according to any one of claims 1 to 3, characterized marked »that the pressure of the gas or gases cyclically with if the frequency is changed by at least 2 cycles per minute. 5. The method according to any one of claims 1 to 4, characterized characterized in that a halide activator has low volatility used at the temperature of diffusion coating formation will. ß09 & 42/1052 26U613 6. The method according to claim 5, characterized in that aluminum is used as the outer material and aluminum fluoride as the halide activator. 7. The method according to any one of claims 1 to 6, characterized characterized in that the article is out of contact with the particle feed bed is held. 8. The method according to any one of claims 1 to 7, characterized in that the particle insert bed is a carrier for Contains recording of the coating material. 9. The method according to claim 8, characterized in that that a refractory oxide support is used. 10. The method according to claim 9, characterized in that that a support made of alumina is used. 11. The method according to any one of claims 1 to 10, characterized in that the halide activator is synthesized in the chamber will. 12. Application of the method according to claim 6 for coating an article made of a nickel-based alloy. 13. Application of the method according to claim 6 for coating an object made of an iron base alloy. 14. Use of the method according to claim 6 for coating an article made from a cobalt-based alloy ring. 15- device for performing the method according to bü9842 / 10S2 26U613 one of claims 1 to 14, which comprises a refractory furnace tube, a lower closed one thereof for receiving the to be covered Object and the particle bed serving the end surrounding heating tube and a gas-tight seal of the upper, open end of the furnace tube, characterized in that the gas-tight Closure (plate 19) is penetrated by a tube (20), by means of which the furnace tube (9) with an auxiliary device (22-27) is connected to continuously and cyclically change the pressure in the furnace tube (9). 16. The device according to claim 15, characterized in that that the auxiliary device in a branch of the pipe (20) has a needle valve (22), a control valve (24) and a protective gas container (26) and in another branch a needle valve (23), a control valve (25) and a vacuum pump (27) contains. 17. Apparatus according to claim 15 or 16, characterized in that that in the upper part of the furnace pipe (9) not surrounded by the heating pipe (5) a concentric, downwardly open, up to two small bores closed nickel tube (8) is arranged, at the closed, with a Nickel mesh (11) covered end of the gas-tight closure (Plate 19) penetrating water cooling pipe system (17, 18) is connected. 18. Device according to one of claims 15 to 17 for performing the method according to claim 7, characterized in that that in the lower, closed end of the furnace tube (9) a cage to be coated with the object (1) is arranged, which separates the article from the particle bed (2) and on the sides and on the upper end surface made of imperforate material, e.g. B. nickel sheet or plate, and at its base. From a gas-permeable wire mesh or network. 609842/1052 Empty page