JPS6132392B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides the above-mentioned method for imparting corrosion resistance at high temperatures to a metal substrate to achieve sufficient performance and long life, particularly in metal products exposed to high temperatures during use. The present invention relates to a method of forming a coating on a metal substrate.

There are many uses for metal products that are exposed to high temperatures. For example, applications include flight space applications and land-based applications such as products used in gas turbine engines.

In these applications, it is important to provide means to prevent unnecessary corrosion of the product. This is because such corrosion shortens the material life of the product and poses significant performance and safety problems. Various alloys, including most superalloys, are characterized by their degree of corrosion resistance. However, when unprotected superalloys are exposed to certain system operating temperatures, corrosion resistance is significantly reduced. For this reason, such products are often provided with protective coatings such as aluminide coatings, thereby increasing their corrosion resistance at extreme operating temperatures.

The aluminide coating is formed by a pack cementation method. In this method, the substrate chemistry and processing temperature greatly influence the coating chemistry, thickness, and properties. In particular, if the coating consists of a hard, brittle outer layer and a hard, brittle multiphase lower layer, these layers may crack at operating temperatures. As a result, fatigue properties deteriorate, and cracks also reduce the corrosion resistance of the product.

Other types of coatings are MCrAlY coatings (M is Fe, Co
or a transition metal element such as Ni). These coatings are currently available on superalloy surfaces.
It is formed by vacuum deposition of MCrAlY alloy.
Such vapor deposited coatings have been shown to have certain advantages over aluminide coatings in providing longer life to turbine products. Unfortunately, such coatings contain radially oriented defects that are formed during the deposition process.
This defect becomes a site of corrosion at high temperatures, leading to premature deterioration of the coated product. Furthermore, the formation of vapor deposited coatings is relatively expensive and requires relatively expensive manufacturing equipment.

So far, several low-cost methods have been investigated to apply MCrAlY coatings on superalloys, such as plasma spraying and slurry sintering. However, most of these attempts resulted in porous coatings that deteriorate prematurely due to corrosion.

It is an object of the invention to provide a metal product which exhibits particular corrosion resistance under high temperature operating conditions.

A more specific object of the present invention is to provide an improved method for treating superalloys exposed to high temperatures, thereby imparting corrosion resistance to the product under high temperature conditions.

Another object of the present invention is to provide a coating, such as a coating, to a metal product, thereby making it possible to use the coating, which exhibits high corrosion resistance at high temperatures, without becoming brittle or cracking;
As a result, the object is to provide a method that allows the physical properties of the product and its corrosion resistance to be maintained at a high level during use.

Yet another object of the invention is to provide a high integrity plasma sprayed metal coating for increased corrosion protection and ductility.

These and other objects will become apparent below, but the invention will be explained in more detail by means of the accompanying drawings, which are given by way of example and not by way of limitation.

FIELD OF THE INVENTION This invention generally relates to a method of applying a coating onto a metal product to impart corrosion resistance to the product at high temperatures. The first step in the invention is characterized by the presence of pores, cavities and similar defects, and some of these defects reach the surface of the coating, and these defects reduce the corrosion resistance of the coating. The second step is to plasma spray the MCrAlY coating (M is one selected from the group consisting of nickel, cobalt, and iron) on the superalloy substrate. The third method is to cover and seal defects that reach the surface of the coating by the outer layer.
The process involves subjecting the coated substrate to hot isostatic pressing at sufficient pressure and temperature and for a sufficient period of time.
Closing internal defects in the MCrAlY coating and defects across the surface and diffusing at least a portion of the metallic outer layer into the MCrAlY coating; The goal is to improve oxidation and corrosion resistance. The coating layer formed according to the present invention has a high degree of integrity, making it suitable for performance at high temperatures.

The MCrAlY coating used in the present invention, a ductile metal coating layer, is applied directly to the surface of the product.
This metal coating layer is formed by plasma spraying. In this case, the metallization layer material is heated to a highly plastic or molten state, which allows a combination of wetting or deformation of the deposited particles as the particles proceed to impinge on the substrate surface. Do it like this. Plasma spraying is particularly desirable for the following reasons. That is, it is generally a low cost technique for forming metallization layers and is applicable to all coating compositions.

The metallization layer obtained in this way generally increases the corrosion resistance of the product at high temperatures due to its coating composition. However, this coating is characterized by having pores, voids and similar defects to a degree or porosity that adversely affects such high temperature properties. And some of these defects reach the surface of the coating.

The method according to the invention includes the step of forming a metallic outer layer on the metallization layer. This outer layer is also made of a material that exhibits corrosion resistance at high temperatures. This material, like the metallization layers already mentioned, shows certain insufficiencies from the point of view of corrosion resistance at high temperatures when used as the only coating on a product. An aluminide coating may be tried as the outer layer. Such coatings, when applied directly to a substrate, tend to exhibit embrittlement and/or cracking phenomena, with the result that they are of minimal use as corrosion protection coatings.

In addition to aluminide coatings, other outer layers such as noble metals and alloys thereof may be used in the method according to the invention. These metals or alloys can be used in combination with the metallization layers described above. This combination eliminates the problems encountered when forming either the gold overlying layer material or the outer layer material alone on a given substrate.
Such a situation occurs when isostatically hot pressing a product with a metallization layer and an outer layer. Gold, palladium, platinum, rhodium may be used as noble metals suitable for the practice of this invention.

In the case of aluminide coatings, the outer layer may be formed by pack cementation or other known techniques (eg, dipping, spraying, metallizing, electrophoresis). If a noble metal is used to form the outer layer, known techniques such as plasma spraying, ion plating, electron beam or vacuum evaporation, sputtering, slurry sintering or pressure welding may be used.

The conditions for hot isostatic pressing may be determined by comparison with the conditions recommended for the substrate. Hot isostatic pressing is thus recommended for superalloys or other materials used at high temperatures, and especially for removing defects that form during casting. Generally, in such techniques, pressures on the order of 10,000 to 50,000 psi are provided by the gaseous atmosphere. The temperature in the autoclave used for hot isostatic pressing is generally the gamma prime solvus temperature of the casting.
The solidus temperature of the casting is increased from 50〓 lower than the prime solvus temperature.
temperature).

When using an aluminide outer layer, the presence of aluminum under hot isostatic pressing results in enrichment of the underlying coating layer. In addition to this, during hot isostatic pressing the base substrate element (nickel in the case of a nickel-based alloy substrate) selectively diffuses outward and becomes found in the coating layer. This diffusion changes the chemical composition of the MCrAlY cover layer and the outer layer. A secure system is thus provided. There is less tendency for cracks to occur in the aluminide layer. This is because the aluminide layer is held together by a ductile, solid (defect-free) layer and not by a conventional brittle multiphase layer. If a crack were to form in the outer aluminide layer, its propagation would be limited by the ductility of the metallization layer. Extensive oxidation of the metallization layer does not occur. This is because the fully dense and chemically modified MCrAlY coating layer resists oxidation and/or corrosion.

If noble metals are used, the advantages mentioned above are still obtained. Therefore, the tendency of the noble metal to embrittle or crack when formed directly on a substrate can be eliminated by interposing a metallization layer and subsequent hot isostatic pressing.

By forming the two layers, the product is encapsulated and surfaces leading to defects in the product are not exposed to the high pressure atmosphere during hot isostatic pressing. As a result, the coating acts as a means to eliminate defects leading to the surface. That is, as mentioned above, the temperature and pressure of the hot isostatic press moves the metal to a range that eliminates the defects.

The coatings described here are characterized, in addition to corrosion resistance, by fatigue resistance and ductility at high temperatures when subjected to hot isostatic pressing. This is a necessary feature of the coating from the point of view of use. Therefore, nickel-based and cobalt-based superalloys,
The diffusion-strengthened alloys, composites, and directional eutectics used in the process of this invention are used where high temperature fatigue resistance and ductility are critical factors.

As already mentioned, the optimum composition of the coating layer consists of a base material of cobalt, iron or nickel and additives of aluminum, yttrium and chromium. Aluminum, initially included in the coating layer or from the outer layer of aluminide, forms Al ₂ O ₃ to aid in oxidation resistance. Yttrium and equivalent additives promote oxide deposition and chromium promotes the formation of Al ₂ O ₃ while imparting hot corrosion resistance.

Aluminide coatings, when used alone, will not provide sustained long-term oxidation, sulfidation, and thermal fatigue resistance. These coatings typically have a marginally ductile continuous phase that tends to crack under high corrosive stresses. When a crack occurs, an oxidizing or other hot corrosive atmosphere can gain access to the underlying substrate.
As already mentioned, such problems are solved by the presence of an intermediate coating layer in combination with hot isostatic pressing. Therefore, the benefits of an aluminide layer can be obtained without the difficulties previously encountered.

By using the coating layer, it becomes possible to effectively introduce elements such as yttrium, which has been difficult to incorporate into the nickel aluminide coating. Such elements are premixed into the coating layer, and in addition, when a coating layer is used, a wider range of nickel and aluminum compositions within the aluminide layer is possible. This avoids traditional limitations regarding the mechanical properties of aluminide coatings.

Next, examples of the present invention will be described.

Example 1 A typical nickel-based superalloy used in gas turbine engines was coated with a coating layer of CoCrAlY. This superalloy, known as IN792+H (trade name), nominally contains 0.15% C, 12.22% Cr, 9.04% Co, 1.97% Mo,
3.97% W, 3.92% Ta, 3.88% Ti, 3.57%
It had a composition of Al, 0.85% H, 0.01% B, 0.10% Zr, and the balance Ni. The nominal composition of the coating layer is 23% Cr, 13% Al, in weight percent
0.6% Y, and the rest was cobalt. This coating layer was formed by plasma spraying. The spray powder was sprayed using a high speed gun (Matsuha 3) operating at 76 KW and using argon and helium as main and auxiliary gases, respectively. The application is
The experiment was carried out in a chamber maintained at a pressure of 50 Torr.
A summary of the parameters of plasma spraying is as follows.

Gun Distance from Sample 16 inches Primary Gas (Argon) V 600 CFH P 250 psi Auxiliary Gas (Helium) V 150 CFH P 250 psi Voltage 85 Volts Current 900 Amps Powder Flow 0.1 lb.PM Carrier Gas (Argon) 50 CFH The covering layer is aluminized by a pack cementation method. This method is disclosed in Freeman et al., US Pat. No. 3,625,750, issued December 7, 1971.
Aluminum sources include 35% aluminum oxide, 67% chromium/aluminum alloy, and 0.02
% to 0.05% ammonium chloride. This method works from 1900〓 to 1950〓
The experiment was carried out under reduced pressure. The aluminum treated coating layer thus obtained was heated to 2200°C in an argon atmosphere.
Hot isostatic pressing was carried out for 2 hours at a temperature of 15 ksi and a pressure of 15 ksi.

A 500x photomicrograph of the plasma sprayed CoCrAlY coating in its unetched state is shown in FIG. High porosity (5% of volume)
It is visible to the naked eye in a coating consisting of an intimate mixture of CoAl (β) and Co solid solution phase (γ). Figure 3 shows a 500x photomicrograph of a plasma sprayed, aluminized, hot isostatically pressed layer. This layer had almost no pores. Tests were also conducted on samples of plasma sprayed and hot isostatically pressed CoCrAlY, where a significant amount of porosity was observed. No pores were observed in the mix where the aluminide coating was prepared as an outer layer of the coating layer of CoCrAlY, indicating that hot isostatic pressing was effective in removing pores only after the aluminide coating was formed. ing.

Another microstructural change that occurs when a plasma sprayed coating is aluminized and hot isostatically pressed is a change in the chemical composition of the coating. Figures 4 and 5 show Al, Co, and Cr for the IN792+H substrate after plasma spraying (Figure 4) and the IN792+H substrate after plasma spraying, aluminum treatment, and hot isostatic pressing (Figure 5). , showing the electron microprobe trace (chemical composition) of Ni element. As can be noticed from these figures, due to aluminization and hot isostatic pressing, the aluminum concentration gradient ranges from about 35% by weight at the outer edge of the coating to about 5% by weight between the coating and the substrate. The range is between. or,
A wide range of amounts of nickel were diffused into the interior of the coating, ranging from 10% by weight at the outer edge of the coating to 40% by weight between the coating and the substrate. This diffusion of aluminum and nickel is caused by (Co, Ni) Al and (Co,
The concentrations of chromium and cobalt elements are varied depending on the thermodynamic stability of Ni) with the solid solution phase. Therefore, a wide range of changes in the chemical composition of plasma sprayed CoCrAlY coatings is achieved after the aluminization and hot isostatic pressing steps.

The performance of products coated according to the present invention was evaluated using 0.7 Mach burner rig testing. The cycle of this test is 1750〓/2 minutes,
1450〓 / 4 minutes, 1750〓 / 2 minutes, air cooling / 2 minutes while injecting 5 ppm salt into a flame containing 0.2% sulfur. With such a test,
The sulfation phenomenon is accentuated and the protective structure and surface oxides are subjected to considerable thermal stress. A comparative graph showing the life of various coatings for the test conditions described above is shown in FIG. The burner rig life of samples treated according to the present invention is approximately five times longer than typical aluminide coatings or coatings;
It has also been demonstrated that the lifetime is approximately 1.5 to 2 times longer than that exhibited by coatings formed by physical vapor deposition or plasma spraying.

As already mentioned, a considerable increase in the life of the coating or film is due to its resistance to oxidation/corrosion.
Attributable to a large accumulation of aluminum, (Co, Ni)Al phases. This layer is supported by a ductile (Co, Ni) solid solution layer, which makes it particularly resistant to thermal fatigue. In addition, the absence of any defects (pores) in the coating does not leave any short circuit paths for corrosion to occur. Therefore, the protective ability of the coating is due to plasma spraying or plasma spraying and aluminizing CoCrAlY.
It increases more than that due to coating.

A process essentially consistent with that described above may be carried out using, for example, 15-40% by weight chromium, 10-25% by weight aluminum, and 0.01-5% by weight other components selected from the group consisting of rare earths and yttrium. Alternative known coating compositions may be used, such as those consisting of 100% of the total amount of nickel, and the remainder consisting of iron, cobalt or nickel. Examples of other coating materials and methods include U.S. Pat.
No. 3,928,026, and No. 3,961,098.

It will be understood that the above-described embodiments of the invention may be modified in various ways without departing from the spirit of the invention as set forth in the claims.

[Brief explanation of the drawing]

The drawings are for the purpose of explaining the method according to the present invention, in which Figure 1 is a graph comparing the durability at a peak temperature of 1750° for various coatings of nickel-based superalloy, and Figure 2 is a graph showing plasma sprayed coatings. Micrograph of coated matrix (500
Figure 3 is a photomicrograph (500x) of a coating matrix plasma sprayed, aluminized and hot isostatically pressed by the method according to the invention;
Figure 4 shows Al in plasma sprayed CoCrAlY coating.
Graph showing electron microprobe traces depicting Co, Cr and Ni contents in a plasma sprayed, aluminidated and hot isostatically pressed CoCrAlY coating according to the present invention.
FIG. 3 is a graph showing an electronic microprobe trace depicting the and Ni content.

Claims (1)

[Claims] 1. Superalloy substrate made of oxidation corrosion resistant MCrAlY
A method of coating with a coating (M being one selected from the group consisting of nickel, cobalt and iron) which: (a) is characterized by the presence of pores, cavities and similar defects; and Some of these defects reach the surface of the coating, and these defects reduce the corrosion resistance of the coating.
plasma spraying a coating onto the superalloy substrate; (b) sealing the surface of the MCrAlY coating with a metallic outer layer, the metallic outer layer covering and sealing the defects reaching the surface of the coating; (c) subjecting said coated substrate to hot isostatic pressing at sufficient pressure and temperature and for a sufficient period of time to
closing internal defects of the MCrAlY coating and defects across the surface and diffusing at least a portion of the metallic outer layer into the MCrAlY coating;
A method of forming a coating on a metal substrate comprising the steps of: closing the defects and increasing the oxidative corrosion resistance of the coating by diffusion of the outer metallic layer into the MCrAlY coating. 2. The method according to claim 1, wherein the metal substrate is placed in a chamber during hot isostatic pressing. 3. The method according to claim 1 or 2, wherein the metallic outer layer is comprised of an aluminide coating. 4 Aluminum from aluminide coating
MCrAlY is diffused into the coating layer, thereby increasing the corrosion resistance of said coating layer and producing a ductile coating, with an aluminum concentration of approximately 35% by weight at the outer edge of the coating and at the interface with the metal substrate. 4. The method of claim 3, wherein an aluminum concentration gradient is formed in the coating such that the aluminum concentration at is about 5% by weight. 5. The base metal of the substrate is selected from the group consisting of Ni, Co and Fe, and these base metals are diffused into the MCrAlY coating layer and the metallic outer layer to increase the corrosion resistance of the coating. Method described. 6. The method according to claim 3, wherein the aluminide coating layer is formed by a method selected from the group consisting of pack cementation, dipping, spraying, metallizing and electrophoresis. 7. The method according to claim 1 or 2, wherein the metallic outer layer is made of a metal selected from the group consisting of noble metals and alloys thereof. 8. The method according to claim 7, wherein the metallic outer layer is formed by a method selected from the group consisting of plasma spraying, pressure welding, vacuum evaporation, sputtering, ion plating, and slurry sintering. .