Hot corrosion resistant coatings

(54) Designation Objects of hot-strength alloy provided with a silicon-enriched hot-corrosion-resistant coating and method of manufacturing the object (56) Published Publications: US 3,493,476 (204-37) US 4,005,989 (428-651) (57) Summary:

The invention relates to a coated article of hot-hole alloy and a method of making the same. The article is provided with a coating which provides protection against heat corrosion at moderate temperatures (649-927 ° C). A coating based on a metal selected from iron, nickel and cobalt or mixtures thereof and containing chromium and aluminum, yttrium and / or hafnium is applied to the article to be protected. A siliceous surface zone is provided at the surface of the coating. Methods including pack cementation and physical vapor deposition are described to provide the coating.

DB 647289

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The present invention relates to the field of heat corrosion resistant coatings for hot-strength alloys. The coating and method described in the present application can be used to apply coatings that are resistant to heat corrosion at moderate temperatures. Such coatings can be used in gas turbine engines and in certain industrial applications.

Materials in gas turbine engines must have both exceptional mechanical properties at elevated temperature and resistance to surface degradation such as oxidation and heat corrosion at elevated temperatures.

In current gas turbines, temperatures are above 1371 ° C, and it is desirable that such engines can operate for periods of more than 10,000 hours without undergoing any significant material degradation. In order to meet these objectives, all components of the hot turbine section of such gas turbine engines with protective coatings are transferred.

Such coatings are of two general types, aluminide coatings and surface coatings. Aluminum coatings are prepared by diffusing aluminum into the part to be protected and reaction between the aluminum and the substrate material to form intermetallic compounds. In use, the portion develops an alumina layer which acts as a barrier to prevent further oxidation of the coated portion. Attempts to apply similar silicon-based coatings directly to hot-strength nickel alloys have not been successful, as the nickel-silicon compounds formed have low melting points.

The other major type of coatings are called surface coatings. Surface coatings are inherently oxidant-resistant and do not depend on any reaction with or diffusion into a substrate.

Typical of the more successful coatings are those called MCrAlY coatings where M is nickel, cobalt, iron or mixtures thereof. Such MCrAlY coatings are disclosed in the specification and claims of U.S. Patents 3,542,530, 3,676,085, 3,774,903 and 3,928,026. Such coatings also obtain their resistance to environmental attack by forming a surface layer of alumina.

Other types of coatings have been evaluated and can be used in connection with the present invention. These include MCr type coatings, MCr Al type coatings and MCrAlHf type coatings (U.S. Pat. No. 3,993,454). In all of these coatings, M may be iron, nickel, cobalt or mixtures thereof.

Great effort has been put into developing coatings that provide ambient stability at extremely high temperatures, e.g. U49 ° C, and for such high temperature applications, the MCrAlY type coatings are usually unmatched 453 306.

Some recent work has exposed an unexpected form of corrosion, which occurs at lower temperatures of from about 649 ° C to about 760 ° C and reaches a peak in the range of 704 ° C to 732 ° C.

The basic principles of packing cement coatings, also known as diffusion coatings, are presented in an article by RL Watchell, found in Science and Technology of Surface Coatings, published by Academic Press, NY 1974, p. 105 to 119.

An article entitled Structure and Properties of Silicide Based Diffusion Coatings by H. van Amerongen describes some siliceous coatings and can be found in a book entitled High Temperature Alloys for Gas Turbines, published by Applied Science Publishers, London, 1978, p. 209 to 224.

Another reference to silicon-based coatings can be found in the documents of the Third International Conference on Chemical Vapor Deposition, Salt Lake City, Utah, April 1972, by PC Felix et al, entitled CVD Silicon Coatings for the Corrosion Protection of Turbine Blades.

U.S. Pat. Nos. 3,873,347 and 3,874,901 describe coating systems in which aluminide coatings are applied to products previously coated with an MCrAlY coating.

The present invention relates to a coated article of hot-strength alloy as well as a method of manufacturing the article. The coated article has a unique resistance to heat corrosion in the relatively low temperature range of 649 to 927 ° C and finds special application in gas turbine engines. The coating comprises a thin adhesive coating on the substrate of a material selected from MCr, MCrAl, MCrAlY and MCrAlHf where M is selected from Ni, Fe, Co and mixtures thereof. The object is characterized in that it additionally comprises a silicon enriched layer at the surface of the coating having a thickness of 10 to 4096 of the total coating thickness and having a silicon concentration of 10 to 50 wt-96, preferably 20 to 40 wt-96.

The invention also relates to a method of making such a coated article, which comprises the steps of providing an article of hot-strength alloy with a coating of a material selected from MCr, MCrAl, MCrAlY and MCrAlHf, where M is selected from Fe, Ni, Co and mixtures. thereof, and characterized by allowing silicon to diffuse into the coating surface to form a silicon-rich surface zone having a thickness of from 10 to 40% of the total coating thickness and containing from 10 to 50% by weight Si, or applying a second coating Over the first coating, the second coating having a thickness of from 10 to 40% of the total coating thickness, the second

453 The 306 coating contains from 10 to 50% by weight Si, and the rest of the composition is similarly a material selected from MCr, MCrAl, MCrAlY and MCrAlHf coatings.

The siliceous portion of the coating can be prepared chemically, such as by a pack cementation process, or by vapor deposition, such as vapor deposition with electron beam.

In order that the invention can be fully understood, reference is made to the accompanying drawing, which shows a photomicrograph of a surface coating with a siliceous surface zone according to the present invention.

Thus, the present invention provides a composite coating which can protect objects of hot-strength alloy against oxidation and corrosion, especially in corrosive environments at moderate temperatures, e.g. 649 927 ° C. In the following description, all compositions are given in percent by weight, unless otherwise specified.

For aircraft engine and other applications where heat fatigue is a problem, the coating should have a thickness of about 25.4 to 254 microns, while for other uses, coating thicknesses of up to 508 or even 1270 microns may be used. The siliceous layer should comprise from 10 to about 40% of the total coating thickness.

The coating according to the invention is based on iron, nickel or cobalt or mixtures thereof. These coatings include, but are not limited to, MCr coatings where chromium varies from about 20 to about 45%, MCrAl coatings where chromium ranges from about 15 to about 45%, and aluminum varies from about 7 to about 15%, MCrAlY. coatings where chromium varies from about 15 to about 45%, aluminum varies from about 7 to about 20%, and yttrium varies from about 0.1 to about 5%, and MCrAlHf coatings where chromium varies from about 15 to about 45%, aluminum ranges from about 7 to about 15% and hafnium ranges from about 0.5 to about 7%. In all of these coatings, M is selected from the group consisting of nickel, cobalt, iron and mixtures thereof, with mixtures of nickel and cobalt being particularly advantageous.

Of the aforementioned surface coatings, those containing aluminum are usually most resistant to high temperature oxidation. Accordingly, the aluminum-containing coatings are preferred if the coating is intended to be used in environments where oxidation at high temperatures (above 816 ° C) may be a problem.

Minor additives of other elements can be made to all these previously described coatings without affecting their basic appearance or their utility as part of this invention.

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Since nickel-silicon compounds have lower melting points than iron and cobalt-based silicon compounds, it is preferred that the nickel content be relatively low, where the coated article is intended to be used in a high temperature environment (higher than about 982 ° C). Preferably, nickel constitutes less than 50% of the W component, where high temperatures are expected.

A wide variety of application techniques are available to make these coatings. These include physical vapor deposition and plasma spraying or other high temperature powder application methods.

We have achieved favorable results with physical vapor deposition, and this is the preferred application technique. The physical vapor deposition technique is described in connection with the application of the MCrAlY coatings in U.S. Patents 3,542,530, 3,676,085, 3,754,903 and 3,928,026.

Briefly, the physical vapor deposition process comprises arranging a melt bath of suitable composition in a vacuum chamber and holding the article to be coated over the melt bath while heating the molten material to cause vapor formation. The steam condenses on the object to be coated.

Usually, the article to be coated is also heated to improve adhesion of the coating and rotate or otherwise handle it above the bath surface to improve the distribution of the coating.

After applying a coating, several post-coating treatments are possible. A conventional process technique particularly useful in MCrAlY coatings includes glass bead blasting for compressing the coating followed by four hours of heat treatment at 1080 ° C to promote the adhesion and homogeneity of the coating. Such a process is described in U.S. Patent 3,528,861 with respect to iron-based coating alloys. Such post-coating heat treatment has not been found to be necessary in the present invention, but does not appear to have any adverse effects.

If plasma spraying or other similar coating techniques are used, the resulting coating could have a higher degree of porosity, and in this case, the various post-coating treatments, such as glass bead blasting and heat treatments, could advantageously be used to reduce the degree of porosity.

After the application of the coating and possible heat treatment after the coating, the silicon-rich surface layer is then developed. Two basic application techniques are envisaged.

In the first application technique, pack cementing, a silica-rich granular pack material is used. The packing material contains silicon carbide, silicon metal or a silicon-rich alloy, an activator such as NH NHCl, NH₂F or NaCl and an inert filler such as alumina. The portions to be coated are embedded in the packing material and then heated at a temperature of from 816 to 1093 ° C for a suitable period of time, the silicon being transported by the activator to the surface of the coating to form the desired silicon-rich layer. During packing, the silicon diffuses into the coating material to form a siliceous layer.

A similar coating technique, but which can be more easily adapted to give a wide selection of different coatings, is physical vapor deposition, the same procedure as previously described with respect to the application of the coating.

In this process, a bath of molten silicon is provided in a vacuum chamber, and the article to be coated is held over the silica bath, where it comes into contact with the silica vapor condensing on the article, which is preferably heated. The condensed silicon diffuses into and reacts with the article as described above for the pack cementation process.

In the current silica deposition coating method used, it was found that by holding the surface coating portion over the molten silicon bath for a period of 15 minutes, deposition of 12.7 microns of silicon was obtained.

During this time, the substrate temperature was maintained at about 95 ° C to promote diffusion and coating adhesion. After deposition of the silica-rich surface layer, the coated portion was heat-treated at 1010 ° C for either 15 or 30 minutes, so that further diffusion of the silicon into the coating was favored. A more adherent coating, about 25 microns thick, was obtained after 30 minutes, which is the preferred treatment time.

It is also envisaged that this vapor deposition technique could be used to apply a silicon-rich alloy to the surface of the previously deposited coating. Such a silicon-rich surface alloy would contain about 10 to about 50% silicon with the remainder of the alloy rich in nickel, cobalt or iron or mixtures thereof and preferably have a similar composition to the underlying coating. Ether, nickel content is preferably kept to a minimum if high temperature operating conditions are foreseen.

The invention can be better understood by reference to the figure. A hot-strength nickel alloy with a nominal composition of 9% chromium, 10% cobalt, 2% titanium, 5% aluminum, 0.15% carbon, 12.5% ​​tungsten, 1% niobium, 0.015% boron and 2% hafnium was provided with a 132 microns thick coating of an alloy consisting of 18% chromium, 12.5% ​​aluminum, # 6% nickel, 23% cobalt and 0.5% yttrium. '

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A physical vapor deposition technique was used to apply silicon to the surface of the coating to form a 25.4 micron thick siliceous layer. This is clearly seen in the figure.

An electron beam micro probe was used for analysis of the major constituents of the resulting coating at various points, and the following results were obtained:

a. Silicon enriched outer layer: 24% silicon, 14% chromium, 43% nickel, 1.9% aluminum, and 16% cobalt;

b. Silica-enriched reaction zone: 20% silicon, 11% chromium, 44% nickel, 6.4% aluminum and 18% cobalt;

c. a silicon + aluminum + chromium-enriched zone: 12% silicon, 21% chromium, 36% nickel, 15% aluminum and 15% cobalt; and

d. base coating; silicon less than 0.2%, 18% chromium, 46% nickel, 12.5% ​​aluminum and 23% cobalt.

These compositions correspond to the points indicated in the figure. It can be seen that the silicon decreases with increasing depth below the surface, and some changes in the composition occur as a result of the combined inward reaction of silicon and the outward reaction of the constituents of the coating.

Of course, those skilled in the art will recognize that the entire article need not be coated. In some applications, only a portion of the article may be subject to corrosive attack, and only these portions need to be coated. Various masking techniques known in the art can be used.

The part of the coating that is not rich in silicon seems to be important for the coating result. Alloys based on iron, nickel and cobalt which contain large amounts of silicon are brittle. Fragile coatings are prone to cracking during operation, and there is always concern that coating cracks may spread inward and eventually cause rupture of the substrate. The previously described silicon-free coatings are relatively extensible, and it is believed that any cracks that occur in the silicon-rich zone will be difficult to spread through the extensible layer to reach the substrate. Silicon has also been observed to be detrimental to nickel-based hot-strength alloys, as it causes the formation of weak, brittle, molten phases. By introducing the coating between the silicon-rich zone on the substrate, these possible adverse reactions between the silicon-rich layer and the substrate are minimized.

Example 1

A hot-strength nickel base alloy is coated with a 101.6 micron coating of NiCoCrAlY material containing 15% chromium, 25% cobalt, 11.5% aluminum, 0.7% yttrium and the residual nickel. The previously described electron beam deposition technique was used to apply silicon to the surface thereof

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NiCoCr AlY coating. The conditions were such that a 25.4 micron thick surface layer containing about 22% silicon was formed.

A test sample with this coating system was tested at 704-732 ° C in a hot corrosion test using a burner device with sea salt and SOg additives to accelerate the corrosion. A non-silicon alloy was also tested with NiCoCrA1Y (18% Cr, 24% Co, 14% Al, 0.2% Y, the residual nickel) coated sample of hot strength alloy and a non-silicon alloy sample of hot strength alloy coated with a silicon-containing NiCoCrAlY composition (18% Cr, 24% Co, 10.3% Al, 0.1% Y, 2.6% Si, the residual Ni) containing 2.6% silicon homogeneously distributed therein as described in U.S. Patent 4,034,142 . The results were as follows: after 470 hours, both the non-siliconized NiCoCrAlY samples had completely failed. Corrosion attacks penetrated the entire 101.6 microns thickness of the coatings and caused substrate damage. After 1800 hours, non-siliconized NiCoCrA1Y showed only about 2.7 microns of infestation. Taking both the amount of corrosion attack and the time involved into account, the silicon alloy reduced the degree of corrosion by about 340%. This clearly shows the surprising and unexpected advantage of the present invention to withstand heat corrosion at intermediate temperatures.

Example 2

Cast samples of Ni-30Cr-8AI alloy were prepared. The cast Ni-30Cr8Al alloy sample represented the coating, and there was no substrate of hot-strength alloy. A sample was siliconized using a physical vapor deposition process to form a siliceous surface layer having a thickness of 15.24 to 22.86 microns and a silicon concentration of 10-16%. This silicon alloy sample was tested in a burner device at 899 ° C along with a non silicon alloy sample. Additions of sea salt and SO 2 were made to increase the severity of the test. The results were as follows: after 353 hours of testing, the non-siliconized sample had corrosion attack to a depth of 2032 microns. The silicon alloy sample showed only 17.78 microns of attack after 1521 hours of testing. If you take the test time and degree of attack into account, you can see that the silicon-alloyed surface layer reduces the corrosion attack by 400-500 times under these test conditions.

Example 3

A test sample of hot-strength alloy with a nominal composition of 8% Cr, 10% Co, 1.0% Ti, 6% Al, 1% Mo, 4.3% Ta, 0.1% C, the residue Ni, was provided with a coating with a composition of 15.5% Cr, 10.7% Al, 0.4% Y, the residue Co. Another test sample of the same alloy was provided with a similar coating with a composition of 18% Cr, 12% Al, 0.5% Y,

453 306 residue co. Both coatings were applied by a physical steam deposition method with electron beam.

A second coated sample was siliconized using a pack cementation process. A packing material consisting of 19% Si powder, 1% NH 2 Cl and the residue AlgOg was used. The coated sample was embedded in this packing material and heated at 871 ° C for six hours. The sample was then removed from the cementing agent and heated to 982 ° C for four hours in an argon atmosphere. The resulting silica-rich surface layer contained about 25% Si and was about 25.4 microns thick.

These coated samples were evaluated in a burner device operated at 704-732 ° C with sea salt and SOg additions to accentuate the corrosion. The following results were observed: after 36 hours, the non-silicon alloy sample showed 55.88 microns of corrosion attack, while the silicon alloy sample after 599 hours showed only 44.45 microns of attack. In this example, the silicon alloy surface reduced the corrosion attack by 20 times.