DE3447469A1 - METHOD FOR PRODUCING POROUS METAL STRUCTURES BY THERMAL SPRAYING WITH A METAL AND A VOLATILE MATERIAL - Google Patents

Description

Process for the production of porous metal structures by thermal spraying with a metal and a volatile one

material

The present invention relates to a method for producing porous metal structures by thermal Syringes such as plasma arc sprayers.

Porous metal structures are made in a variety of processes and used for different situations. It has been found that porous metal structures are particularly useful for abradable seals, by which structures are to be understood that are easily removed at great speed when they are removed by one High-speed part are touched, but otherwise retain their integrity. You become special used in the field of engine construction. For the prior art, reference is made to US Pat. No. 4,049,428 (Elbert et al) and U.S. Patent 3,111,396 (Ball).

One of the preferred methods of making porous metal structures is to use a compact or to produce compact bodies made of metal and a volatile material and then to produce the volatile material remove so that a metal structure with a density remains below full density. Conventional powder metallurgy techniques have been used which include the Preparation of a mixture that includes pressing and sintering. On this prior art, see US Pat. No. 3,864,124 (Breton et al), U.S. Patent 3,897,221 (Salyer et al) and the patents cited above. Also see U.S. Patent 3,350 (Miller) to refer.

Due to the substantial shrinkage caused by sintering in a powder metal article, we have so far used plasma arc spraying for

Position of porous metal structures preferred. During the plasma spraying of a metal-polymer mixture, the metal particles bind together. This either eliminates the need for subsequent sintering or, if carried out, does not result in excessive shrinkage. Generally speaking, a composite article was first produced according to the teaching of US Pat. No. 3,723,165 (Longo et al), according to which a metal powder such as, for example, nichrome, together with a high-temperature polyester powder such as, for example, poly (paraoxybenzoyl), which has a high melting point is splashed. The polyester softens but does not melt during spraying. Although the specific step of creating a porous structure by subsequently oxidizing the polymer is not mentioned in the referenced patent, we and others have performed this step to produce test porous seal structures for gas turbine engines. As the polyester used is a high temperature material, temperatures in the range of 540 ⁰ C are required to oxidize away the volatile ingredient. Although the abradable seals made in this way are effective for gas turbine engine use, the cost of the particulate polyester resin is high. Improvements have therefore been sought which both reduce cost and improve the performance of abradable porous metal seals by changing their physical and chemical properties.

It is an object of the present invention to provide an improved method for making porous metal structures, particularly for making an abradable seal for a turbine engine _; indicate this leads to a seal with improved performance at lower

35 cost leads.

This object is achieved by a method according to claim 1 or 10. Advantageous refinements are the to be found in the dependent subclaims.

Thus, according to the present invention, there becomes a meltable polymer which has a low volatilization temperature comprises a sprayed-on coating, thermally sprayed onto a substrate together with a metal powder. The polymer is volatilized from the pavement by heating to a temperature below that is at which more than 30 wt .-% of oxides are formed in the remaining metal coating. It was found, that a low oxide content in a material for an abradable seal improves its behavior, in particular with a view to avoiding buffing when interacting with a compressor rotor blade.

Preferably, plasma arc spraying or another spraying process that involves excessive oxidation of the particulate material is avoided, is used to make a first coating of a mixture of the Forming polymers with the metal on a substrate. The polymer is then heated to a relatively low temperature removed. The polymer can disappear by forming a volatile one Substance depolymerizes that it is chemically forming of a volatile substance reacts to dissolve or by any other process in which no excessive heat generation occurs. Depolymerization is preferred because it is an endothermic process is and in this way forms an advantageous contrast to an oxidation process in which the temperatures exothermically increased and undesired oxidations or other reactions can be caused. If the present Invention is applied to nichrome alloys,

Oxide contents of 4-30% by weight are produced in structures whose density is 26-40% of the density of the solid metal. Such numbers compare to a typical 30-40 weight percent oxide content characteristic of porous nichrome metals made in the prior art using a high temperature polymer. Preferably, the polymer and metal coating is heated in air to reduce cost. For Nichrome alloys consisting essentially, that one performs the removal process at a low temperature, namely at less than 54O ⁰ C, preferably in the range of 250-430 ⁰ C means.

According to a preferred embodiment of the present invention, 14% by weight of polymethyl methacrylate powder are sprayed together with 86% by weight of 80Ni-20Cr powder. The coating produced in this way is heated to about 315 ° C. in order to convert the polymer into a volatile monomer. The resulting porous metal structure has an apparent specific gravity of 2.7 g / cm ³ or about 32% of the theoretical value for the metal.

Using plastics that are meltable is compared with plastics that are made during injection molding only soften, obtain a pore structure with improved character. This pore structure is with the better The behavior of the abradable seals is just as linked as the lower oxide content.

The above, as well as other objects, features, and advantages of the present invention are apparent from US Pat the following description of preferred exemplary embodiments with reference to the figures even more clearly.

35 It show:

Fig. 1 is a micrograph of a cross section through

a porous metal structure made of nichrome, which was produced according to the invention using polymethyl methacrylate;

FIG. 2 shows a micrograph similar to that in FIG. 1,

however, a nichrome structure is shown which is in accordance with the prior art and using a polyester and the one opposite

Fig. 1 has different porosities;

3 is a diagram showing

like the oxide content the gloss finish

The character of an 80-20 nichrome structure is influenced, the absence of which a gloss finish is good;

Fig. 4 is a photograph of the shape of particulate

shaped polymethyl methacrylate (Lucite) after thermal spraying in the free air; and

FIG. 5, similar to FIG. 4, shows the shape of a

particulate poly (paraoxybenzoyls) (Ekonol).

The present invention is illustrated using the example of manufacture a grindable seal made of a nickel-chromium alloy, 80Ni-20Cr, described. Splashing one Mixture of a metal and a polymer is disclosed in U.S. Patents 3,179,784 (Johnson) and 3,723,165 (Longo et al), reference being made to these patents to supplement the present disclosure. In the preferred embodiment of the present

Invention is used, a polymer powder having a depolymerization temperature of less than 430 ⁰ C. Preferably the powder is a polymethyl methacrylate such as a Lucite Grade 4FNC-99 powder (Dupont Company). A powder mixture of 14% by weight Lucite and 86% Nichrome is sprayed. The nichrome has a nominally -250 +500 mesh (Tyler Sieve Series mesh size) particle size (hereinafter simply referred to as "-250 nichrome") while the Lucite has a corresponding particle size of -80 +400 mesh. About 0.2% by weight of sub-silica particles, such as Cab-o-Sil powder (Cabot Corporation), is added as necessary to improve the flow properties of the mixture. The mixture is passed through a plasma arc torch and deposited to a thickness of about 2 mm on a suitably prepared substrate, for example a workpiece made of a nickel superalloy, using the usual conditions for plasma arc spraying.

After the desired thickness of the sprayed-on covering has been produced, it is heated in air, in a vacuum or in an inert atmosphere. Air is preferably used for the sake of simplicity. The article is heated to a temperature of about 315 ° C for about 2 hours to cause depolymerization of the polymer and generate the monomer which volatilizes from the article, thereby forming an on the substrate porous metal structure remains. 1 is a micrograph of the cross section of the layer of nichrome metal obtained, which has an apparent density of 2.7 g / cm ³ , which corresponds to about 32% of the theoretical density for nichrome metal and Fig. 2 appear essentially the same as shown when viewed perpendicular to the cross-section). Density is conventionally measured by having the

Dividing the weight of a sample by the volume enclosed by its outer surface. However, the porous nichrome structure described above is comprised of metal and about 25% metal oxide. The Ni-Cr oxides, which were not properly characterized, have a lower specific gravity than the metal. Thus, the actual porosity of a structure made in accordance with the present invention is less than the ratio of the specific gravity of the structure to the specific gravity for the fully dense metal; For example, the 2.7 g / cm ³ pore structure of 8.4 g / cm ³ nichrome, which corresponds to an apparent density of 32%, is slightly below 68% with regard to the cavity structure.

To understand the advantages of the present invention, it should be noted that the desired function of a Abrasive material consists in the fact that it is subject to erosion by particles and other mechanical stresses remains intact. Even so, it must easily crumble in a brittle manner if it moves at high speed moving part, such as a blade tip, is touched. This property is called "abrasion When such easy disintegration behavior is absent, the tip of the blade becomes excessively heated and self-destructive. In behavioral tests involving operation in a gas turbine engine simulate, the structure produced using the polymethyl methacrylate resin turned out to be Surprisingly as different and superior to those made using polyester resin were manufactured. It is particularly noteworthy that the new abradable material has a lower tendency shows smudging or being polished to a glossy finish when touched by the blade tip, in comparison with the same structure when it was made using a polyester resin.

A gloss finish is symptomatic of an inadequate one Abradability. We have improved some of this Behavior uncovered phenomena underlying the use of meltable low-temperature polymer powders, such as, for example, made of polymethyl methacrylate.

First, it has been found that the oxide content of the improved abradable seals is about 25% by weight using the -250 nichrome powder, as compared to the same seal made using the polyester resin, in which case the Oxide content is about 35% by weight. Although experience appears to suggest that higher levels of oxide are desirable because they are generally associated with embrittlement of metals, we have found that gaskets with lower levels of oxide perform better. Fig. 3 shows the dependence of the occurrence of polished sections on the oxide content for a particularly useful density range of nichrome. The data obtained are based on the visual examination of the abraded surface of a porous metal structure, which at a temperature of about 24 ⁰ C of six simulated AMS 4928- Titanium alloy blade tips at abrasion velocities of approximately 290 m / s. A smearing or gloss finish (glazing) can be recognized by the fact that the abraded surface of the sprayed-on structure is shiny and metallic, compared with the dull appearance after the abrasion test. In addition, the polished finish can also be recognized by considerable wear on the rubbing rotor blade, in comparison with a non-glossy ground state in which the aggregate volume of titanium that disappears from a rotor blade is less than 0.5-2% of the aggregate volume of the material, which during a

_ ₁₂ _ "3U7469

Abrasion testes is removed from the sealing structure. The oxide content of a gasket structure is under application conventional dissolution techniques calculable. For Nichrome we use a hot methanol-5% bromine solution and characterize the insoluble residue as an oxide.

When removing the polyester resin from a sprayed-on covering, known from the prior art, the ovens must be set to about 540 ^° C. because of the high-temperature properties of the resin. It has been found that the oxidation of the polymer, the temperature of the porous metal structure further raises to exothermic manner during such a distance to about 62O ⁰ C. Because of the high surface area of a porous metal structure, the relatively high degree of oxidation occurs even though nichrome is an oxidation-resistant alloy. It has thus been found that the method of removing the polymer is critical and that it is necessary to use a volatile material which will depolymerize or otherwise volatilize at a low temperature. The oxide is generally dispersed in the metal of the structure and is not, as one might expect, concentrated on the surfaces surrounding the visible pores. Table shows the oxide content of a 2.7 g / cm ³ 80-20NiCr-porous material, which is obtained with 100 h heating to certain firing temperatures, which can be used to drive off various volatile materials.

go nen. The data show an unexpected superiority in the With regard to the oxidation resistance of a covering made with polymethyl methacrylate (Lucite), when compared to the same precipitate made with polyester (Ekonol). One can do that Be a consequence of the more favorable pore structure that is generated by the meltable polymer. Our process makes

it is possible to achieve a desired oxide content of less than 35%, usually in the range of 20-30%. With metals with similar characteristics such as oxidation 80-20 Nichrome this it is necessary in general that the polymer at a temperature of less than about 540 ⁰ C decomposes or is converted to a volatile component such as a monomer or a gas. Lower temperatures are required for metals that are less resistant to oxidation.

Second, the physical structure of the porous metal made using a low temperature meltable polymer is also improved when comparing the abradability with that of a corresponding material made using a higher temperature softening polymer. This can be seen if one compares FIG. 1 with FIG. 2, which shows a micrograph of a cross section obtained by spraying a mixture of 75% by weight NiCr and 25% by weight poly (paraoxybenzoyl) polyester resin Ekonol (Carborundum Co. and Metco Inc.) with a particle size of -150 +325 mesh. The sample shown in FIG. 2 has a density of 2.8 g / cm ³ , which is essentially the same as the sample in FIG 1. (A low separation efficiency makes it necessary to use a higher volume percentage of Ekonol compared to Lucite in order to obtain the same porosity). Nevertheless it can be seen that the structure in FIG. 1 has a greater openness. We attribute this to the behavior of the particulate lucite as it melts and appears to agglomerate during its transfer to the substrate, which the ekonol does not. Melting-type polymers create a larger pore size range, and the larger amount of large pores results in the open

35 ner appearance of the structure.

TABLE 1. Approximate pavement oxygen content from the abrasive material and its influence by a 100 hour Heat

Oxygen content

injected polymer / metal

Lucite / -250 mesh 80Ni20Cr Lucite / -325 80Ni20Cr Ekonol / -250 80Ni20Cr Ekonol / -325 80Ni20Cr

Lucite / -325 80Ni20Cr Ekonol / -325 80Ni20Cr

Lucite / -325 CoCrAlY * Lucite / -325 NiCrAlY *

15 * See U.S. Patents 3,676,085, 3,754,903 and 3,542,530

Heating
temperature.
( ⁰ C) Beginning (% By weight
End 540 7th 11th 540 21 36 540 27 43 540 - 43 650 22nd 47 650 32 58 540 15th 22nd 540 7th 10

»Fc» I

FIGS. 4 and 5 show the difference in behavior of Lucite and Ekonol resins when these through as such Plasma arc syringes are sprayed into the open air and collected. The Ekonol material according to FIG. 5 shows no sign of melting and continues to exist in the form of individual particles while the lucite is closed spherical bodies is melted and agglomerated.

Generally speaking, the invention relates to the use of a polymer which melts and which volatilizes at a temperature below that which results in greater than 30% oxide in the metal. Various polymers are suitable for nichromes, including those selected from the general group formed by polystyrenes, polyethylenes, polypropylenes and polyacrylates, since all of these polymers are removed from the substrate ^{at atmospheric pressure and a temperature of less than 540 ° C.} to evaporate. For example, polymethyl methacrylate decomposes at about 250 ^° C., and for reasons of convenience and to speed up we heat it to about 315 ^° C. Another reason for the preference of the last-mentioned material is its ready availability in particulate form at low cost.

Of course, the primary cause of oxidation of the porous structure which is considered by our process is the step of removing the volatile polymer. However, care should also be taken to prevent oxidation during the thermal spraying process. For a nichrome polymer mixture, plasma arc spraying in air with 50-50 argon-helium at an enthalpy of about 7 kWh / m ³ gives good results. Plasma arc spraying in air is generally applicable since it involves the use of non-oxidizing gases. It can

however, other thermal spray methods, such as flame spraying and methods using a detonation jet gun, are useful if they are known to produce precipitates having relatively low oxide levels of less than about 25 percent. Of course, the oxide content of a covering varies with the size of the metal powder used, with coarser powders leading to a lower oxide content. The use of -250 nichrome results in an oxide content in the range of 4-10% after the polymer has been removed. In this case, not only the freshly sprayed coating of a coarser metal powder less oxide, but we also found that the oxidation rate is lower at a constant temperature in the range of 200 to 650 ⁰ C, apparently due to differences of character of the sprayed structure. In this connection, reference is again made to Table 1, which can be seen that a -325 mesh +500 mesh powder (-325 mesh 80Ni 20Cr ") compared to a high oxygen content

20 leads with a -24 0 mesh 80Ni 20Cr powder.

If a high temperature abradable seal is made, the spray coating will be on a curved one Pieces made of a nickel or iron superalloy, e.g. applied to IN 718 or AISI 410 alloys. After this Once the volatile material has been removed, the porous metal structure is firmly attached to the superalloy substrate, through the sip is attached in the engine, tied. In order to produce such structures from nichrome, we inject from 80-90% by weight nichrome with 20-10% polymethyl methacrylate. These mixtures provide coverings with 35-45% by volume of polymers, preferably 37-43% by volume. The metal coatings obtained have void contents after the polymer has volatilized of 50-70% by volume, i.e. the apparent density

35 is about 30-50%.

As a specific example, the above-mentioned nichrome sample, produced by spraying 86% nichrome with 14% polymethyl methacrylate, has a density of about 2.7 g / cm ³ and an oxide content of 7%. The weight loss measurement during the removal of the volatile polymer shows that the covering consisted of 43% by volume of polymer. The apparent density is 32% so that without the slight correction to account for the metal oxide volume (compared to the pure metal alloy) the apparent void content is 68%. Thus, the void content of a finished pavement is greater than that produced by the polymer.

It is well known that freshly sprayed plasma coatings are porous and this is inherent in voiding as well as other phenomena associated with the process described appear to be wearing help create the finished product.

To get similar results with the other polymers mentioned Proper adjustment of the pavement composition is necessary to maintain the change in density to be considered for maintaining the desired porosity. Another setting of the composition of the sprayed mixture is therefore carried out in order to increase the efficiency of the deposition of the polymer component and the Metal component to be considered.

The present invention has particular relevance to porous abradable sealing structures in general Nichrome alloys, as they are generally known in various compositions, as such materials previously showed good behavior in the seals produced according to older techniques. Nonetheless, it is The present invention is applicable to other materials containing other nickel alloys as well as alloys

1 include the base of iron, cobalt and aluminum.

Although the present invention has been described in the present specification with respect to a preferred embodiment, it will be understood by those skilled in the art that numerous changes in form and details are possible without departing from the scope of the present invention as defined by the claims , is left.
10

Claims (10)

Dipl.-Chem. Dr. Stefren ANORAE Dipl.-Phys. Peter FLACH Dipl.-hng. Dietmar HAUG 07 ",,"., Dipl.-Chem. Dr. Richard KNEISSL Lt 'DEZ · »<" PATENTANWÄLTE SteHnstr. 44, D-8000 Munich 80 Note: United Technologies Corporation Hartford, Ct. 06101, V.St.A. Az: 310 AS / sc Process for the production of porous metal structures thermal spraying with a metal and a volatile material A method of making a porous metal structure which comprises thermally spraying a mixture of metal powder and polymer powder onto a substrate to first create a sprayed-on coating and then heating the coating to volatilize the polymer from the coating ,
characterized in that the polymer powder is melted during spraying so that a desired pore shape is created in the metal structure, and through Heating the sprayed coating to a temperature of less than 540 ⁰ C, to cause an endothermic reaction and the removal of the polymer and to produce an oxide content in the coating of less than 3 wt .-% 0. 2. The method according to claim 1, characterized in that the heating takes place in an oxidizing environment. 3. The method according to claim 1, characterized in that the polymer is selected from the group consisting of polystyrenes, Includes polyethylenes, polypropylenes and polyacrylates. 4. The method according to claim 1, characterized in that the polymer is polymethyl methacrylate. 5. The method according to claim 3, characterized in that the particle size of the metal powder between 200 and 500 mesh and the size of the polymer powder between 80 and 400 mesh (each Tyler Sieve Series mesh size) amounts to. 6. The method according to claim 2, characterized in that the porous metal structure on a metal substrate is deposited in the form of a seal for a gas turbine engine and that the porous metal structure is not is polished to a shine when it is driven by a titanium alloy blade moving at a speed is moved at about 290 m / s, is rubbed. 7. The method according to claim 2, characterized in that the heating to a temperature of 250-430 ⁰ C ₂₅ follows. 8. The method according to claim 2, characterized in that the sprayed-on coating is about 35-45% by volume of polymer contains. 9. The method according to claim 2, characterized in that that the proportion of the polymer is 37-43% by volume. 10. Method of making a porous metal structure for use as a seal in a gas turbine engine that utilizes the thermal spraying of a mixture -3- comprising a metal powder made of a metal alloy and a polymer powder on a substrate in order to first produce a sprayed-on coating, and heating the coating to cause the polymer to volatilize from the coating, characterized by spraying and melting a polyacrylate polymer and of a metal to form a sprayed-on coating containing 3-545% by volume of polymer; to generate and endothermic decomposition of the polymer by heating the sprayed coating to a temperature of less than 540 ⁰ C in air to form a porous metal structure having a desired pore shape and less than 30 wt .-% oxides.