Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-08-31
2004-05-04
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S280000, C148S283000, C427S229000, C427S237000, C427S253000
Reexamination Certificate
active
06730179
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of aluminide coatings diffused onto metal substrates and particularly to targeting the diffusion of the coating to a selected area of the substrate.
BACKGROUND OF THE INVENTION
Diffusing aluminide coatings onto the surface of metal gas turbine components, such as blades, vanes, combustor cases and the like, is a standard way of reducing the untoward effects of oxidation and corrosion on these components, thereby maintaining their useful life. Specifically, aluminide coatings extend the service life of a part used for operation at temperatures usually above 649° C. (1200° F.). Such parts are usually made from nickel or from nickel or cobalt based alloys.
Essentially, all aluminum diffusion coating methods share some common steps for accomplishing the coating: first, the coating material is placed near or in contact with the metal substrate; the coating material and substrate are then heated until the coating material diffuses onto the substrate. More specifically, the placement step involves placing the metal substrate in a retort chamber with a source of aluminum and a halide activator. The source of aluminum may be pure aluminum or an aluminum-rich intermetallic compound such as a chromium-aluminum alloy or CO
2
Al
5
and the like. The activator may be any number of halide compounds, including an aluminum halide, alkali metal halide, ammonium halide, or mixture thereof. The activator functions to facilitate the deposition of aluminum onto the surface of the metal component.
High heat is then applied to the metal substrate, aluminum source and activator in the retort chamber for a period that ranges from two to twelve hours in an inert atmosphere to prevent the occurrence of oxidation. During the heating step, the halide activator dissociates and reacts with aluminum metal ions from the aluminum source to form Al-halide intermediates, which migrate to the surface of the metal substrate. The Al-halide intermediates “grab” the metal atoms of the metal substrate. These atoms reduce the Al-halide intermediates to create intermetallic compounds, such as Ni
2
Al
3
, NiAl or NiAl
3
, on and at some depth below the surface of the metal substrate. These intermetallic compounds are aluminides and are generally resistant to high temperature degradation. They are consequently preferred as protective coatings.
Diffusion aluminide coating methods also share a second commonality, called activity or throwing power, which stems from the use of a halide activator. Throwing power relates to the strength of the halide activator in reacting with the aluminum ions in the aluminum source. Throwing power is essentially a measure of the potential that a halide activator has in facilitating a coating reaction. Those halide activators with greater throwing power form more reactive Al-halide intermediates. Accordingly, they can more readily pull the metal atoms of the substrate out of their crystalline structure as well as pull out metal atoms from deeper in the substrate. Halide activators with greater throwing power are able to facilitate a stronger coating reaction, which in turn relates to the thickness of the deposited coating.
Diffusion aluminide coatings thus depend on the chemical reactivity between the aluminum-halide intermediate and the metal atoms of the substrate, which, as just discussed, is a function of the reactivity of the halide activator. Other factors that affect the depth and quality of the coating include the heating temperature and the presence of any other material placed either in the heating chamber or on the surface of the substrate that could inhibit the throwing power of the halide activator.
Essentially, the differences between the various diffusion coating methods relate to the distance in placement and to the proximal relationship between the coating material and the substrate. Historically, aluminide coatings have been formed by the so-called “pack cementation” method described in U.S. Pat. No. 3,257,230 to Wachtell et al., and U.S. Pat. No. 3,544,348 to Boone. In this method, the metal substrate is buried in a coating material in powder form that contains an aluminum source and halide activator. That is, the coating material has an in-contact relation with the substrate. Other in-contact coating media include coating tape and slurry. Because the media is applied directly to the surface to be treated, these methods represent variants of the pack cementation method. In fact, U.S. Pat. No. 5,334,417 to Rafferty et al. discusses using coating tape to form a pack cementation-style coating on a metal surface. U.S. Pat. No. 6,045,863 to Olson et al. employs a coating tape that produces a two-zone diffusion coating. U.S. Pat. No. 5,674,610 to Schaeffer et al. uses a coating tape to perform a chromium, not aluminide, diffusion coating. U.S. Pat. No. 4,004,047 to Grisik features a coating tape in which the aluminum source is a Fe—Al powder mixture. Also, U.S. Pat. No. 6,110,262 to Kircher et al. discloses a slurry for diffusion aluminide coating.
Somewhat different from the pack cementation method is the so-called “above-the-pack” coating method in which the metal substrate lies in a retort chamber apparatus above the coating material. The coating material is typically in powder form, and has an out-of-contact relation with the substrate. Besides an aluminum source and halide activator, the coating material may contain an oxide and modifier as required to reduce the activity of the halide activator. See e.g., U.S. Pat. No. 4,132,816 to Benden et al.; U.S. Pat. No. 4,148,275 to Benden et al.; U.S. Pat. No. 4,501,766 to Shankar et al., and U.S. Pat. No. 5,217,757 to Milianik et al. Essentially, these references describe vapor aluminide diffusion, whereby internal features of a metal part may be coated. A further variation is the chemical vapor deposition method of U.S. Pat. No. 5,658,614 described in Basta et al.
A problem in the use of diffusion aluminide coating for gas turbine engine parts has been the inability to consistently attain uniform coatings of inaccessible or hard to reach sections of the part to-be-coated. Methods that require in-contact relation between coating medium and the metal substrate cannot coat an inaccessible section, regardless of whether the medium is in powder form, a tape or a slurry.
The amount of coating medium applied to the substrate surface usually affects the diffused coating thickness. Previous in-contact coating methods result in a hit or miss approach to the application of coating medium for hard to for reach sections of the part. However, depending on the geometry and the irregularity of the section to be coated, using an in-contact coating mechanism such as a powder or slurry for hard to reach sections of the part likely results in an uneven coating layer applied to the substrate. In many instances the best that can be done to deliver coating medium to the hard to reach metal substrate is to estimate that an in-relation contact has been made. Further, disposing a slurry on a hard to reach part risks undetected or uncontrollable contact onto sections of the part that ought not be coated. Detecting a spotty or uneven application of the coating medium may be difficult. Moreover, when an undetectably uneven application of coating medium has been heated, detecting a non-uniform coating thickness is difficult.
Aluminide diffusion methods that allow an out-of-contact relation, such as above-the-pack cementation or vapor diffusion, may provide somewhat more control than in-contact methods. This is because in the above methods, diffusion coating occurs as a result of the entire surface of the part being automatically exposed to the aluminum vapor in the heating chamber. For example, relative to hard to reach surfaces, above-the-pack cementation has provided a way to deposit a metallic coating on internal surfaces of hollow articles, such as gas turbine blades and vanes. See U.S. Pat. No. 4,148,275 to Benden et al. Hollow gas turbine blades are placed in a chamber atop that i
Drinker Biddle & Reath LLP
Oltmans Andrew L.
Sermatech International Inc.
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