Vapor randomization in vacuum deposition of coatings

Abstract

In the processes for forming protective coatings on metal substrates, particularly the nickel-base and cobalt-base superalloys, by deposition in vacuum, an inert gas leak adjacent the substrate is utilized to randomize the coating vapor cloud and cause non line-of-sight deposition.

Parent Case Text

This application is a continuation of Ser. No. 806,873, filed Mar. 13, 1969, now abandoned.

Description

BACKGROUND OF THE INVENTION

The present invention relates in general to metal coating processes and apparatus therefor and, more particularly, to vacuum deposition processes.

It is well known that the conventional nickel-base and cobalt-base superalloys do not in and of themselves exhibit sufficient oxidation-erosion resistance to provide component operating lives of reasonable duration in the dynamic oxidizing environments such as those associated with the operation of gas turbine engines. Accordingly, it has been the usual practice to provide these alloys with a protective coating in such applications.

Although the aluminide coatings, such as that described in the U.S. Pat. No. to Joseph, No. 3,102,044, have in the past displayed satisfactory performance, it is well known that these coatings, because of their dependence upon the availability of substrate elements, often are characterized by a composition less than optimum. Furthermore, these coatings are often achieved only at the expense of some mechanical property less in substrate strength.

Many of the more advanced coatings developed for the next generation of jet engines depend in the first instance on the deposition of a high melting point coating alloy with a concurrent or subsequent reaction with the substrate to attain the desired end composition, microstructure or adherence. These new alloys generally demand the application of special coating techniques to provide the right species in the right amounts at the surfaces to be protected.

Several coating compositions of current interest are described in detail in copending applications of the present assignee. Among these compositions is that hereinafter referred to as the FeCrAlY coating at a nominal composition of, by weight, 30 percent chromium, 15 percent aluminum, 0.5 percent yttrium, balance iron, as discussed in the copending application of Frank P. Talboom, Jr., et al entitled "Iron Base Coating for the Superalloys," Ser. No. 731,650 filed May 23, 1968. Another such composition is the CoCrAlY composition at about, by weight, 21 percent chromium, 15 percent aluminum, 0.7 percent yttrium, balance cobalt.

The basic problems associated with the deposition of these coating alloys relates to their high melting points and the difficulty of providing the right amount of all of the alloy species in the coating as applied. Satisfactory results have been attained through the use of vacuum vapor deposition techniques, such as that suggested in the U.S. Pat. No. to Steigerwald 2,746,420. These processes, which have in the past been primarily directed toward the application of relatively low temperature materials of relatively simple composition, are in the present instance characterized by extreme sensitivity to variations in the process parameters and, accordingly, reproducibility as well as processing expense is a problem.

The vacuum vapor deposition of electron beam melted metals in existing low evaporation rate, production-type systems, such as high cyclic speed or strip line coaters, has essentially been limited to line-of-sight coating from the source (molten pool of coating metal) to rotating or linearly moving substrates. Recently, several techniques have been developed to improve the versatility of the basic process through collimation or densification of the vapor cloud. In one such method, a gas cascade or multi-orificed nozzle surrounding the pool of molten coating material is utilized to introduce a high velocity inert gas inwardly at an angle to the vapor cloud to densify the direction of the metal vapor atoms thus permitting increased coating rates of line-of-sight areas. In another such method, a high mass, high temperature reflector is utilized to the same end.

SUMMARY OF THE INVENTION

The present invention contemplates a vacuum deposition process which utilizes controlled inert gas impingement on the vapor cloud to randomize and redirect the coating material vapor cloud. For this purpose, a low velocity, inert gas leak is admitted, to a chamber pressure sufficiently low to prevent sustaining a gas plasma but sufficiently high to substantially decrease the mean free collision path of the metal vapor atoms in the vicinity of the surface to be coated, to randomize the direction of the metal vapor atoms and, in essence, to cause coating of non line-of-sight areas.

BRIEF DESCRIPTION OF THE DRAWING

An understanding of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawing, wherein is shown a schematic illustration, partially in section, of vacuum vapor coating apparatus in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one particular embodiment as illustrated in the drawing, there is shown a vacuum chamber 10 having an exit port 12 leading to a suitable high vacuum pump, preferably of the diffusion type, for the rapid and continuous evacuation of the chamber. Located inside the chamber, there is shown an electron gun 14 for generating a beam of charged particles to impinge upon and vaporize an ingot of source metal 16. It will be appreciated by those skilled in the art that the electron beam is suitably directed by conventional magnetic deflection pole pieces 18. Of course, the arrangement of the electron beam gun within the vacuum chamber is a function of design. A 30 kilowatt electron beam unit has provided satisfactory deposition rates with a two inch diameter ingot of a FeCrAlY coating material the depth of the molten pool usually being 1/4 - 1/2 inch.

The ingot 16 is made movable and is slidably received at its upper end by an annular water cooled crucible 20. The ingot is normally continuously fed upwardly into the crucible through a heat resistant vacuum seal 22 in the chamber wall at a controlled rate by a chuck 24 to maintain a constant pool height.

It is important that the pool elevation be maintained constant not only because the coating efficiency, composition and uniformity are very susceptible to pool height changes, but also so that the focused electron beam will impinge only on the desired pool surface area.

The substrate to be coated is disposed within the vacuum chamber 10 vertically above the ingot 16 and is illustrated as a gas turbine blade 26 having an airfoil section 28 and a shroud section 30. Since the coating process is fundamentally line-of-sight, the part is typically mounted to effect rotation about its longitudinal axis, that is, the longitudinal axis of the airfoil 28, usually utilizing a pass-through (not shown) through the vacuum chamber to an external drive system. Of course, more than one part may be coated at a time. In such a case, in order to minimize non-uniformity of coating between each of the plurality of parts, each part is normally mounted in a plane of vapor isodensity or roughly along an arc defining a zone of constant vapor concentration, the parts closest to the vertical passing through the center of the molten pool being located slightly farther from the pool surface than those positioned at an angle with respect to the said vertical. Whether coating a single part or a plurality of parts however, each substrate is further positioned as close as possible to the surface of the molten source pool for maximum coating efficiency but far enough removed therefrom to prevent coating contamination by splash from the pool. The substrate height varies with each system but for a 2 inch diameter pool and a deposition rate of about 0.3 mils per minute with a FeCrAlY coating material, a mean height of about 10 inches has been found satisfactory.

As mentioned previously, the vacuum vapor coating process is essentially line-of-sight. Although axial rotation of the part is successful in effecting deposition along its entire length, it does not alleviate the problem of coating the remaining end portions. This is particularly unsatisfactory in a substrate having an enlarged end portion such as the shroud 30 of the turbine blade 26. In accordance with the present invention, there is provided an inert gas line 32 adjacent the outer surface of shroud. The line 32 admits an inert gas, preferably helium, at a low velocity from a location generally above and outwardly from the shroud in a direction generally downwardly and inwardly theretoward. The inert gas leak is controlled to a chamber pressure sufficiently low to prevent sustaining a gas plasma yet sufficiently high to substantially decrease the mean free collision path of the metal vapor atoms in the vicinity of the shroud. It is to be noted that the inert gas should not be of such impurity as to cause occluded impurities in the coating. An inert gas having an oxygen and moisture content each less than one ppm has been found satisfactory. In essence, the inert gas leak randomizes the direction of the metal vapor atoms and thus causes them to impinge on and coat an area not line-of-sight with respect to the source.

A number of tests were conducted with various coating materials and various substrate alloys. In one series of tests, argon and helium gas was introduced at a pressure of 17 psi through a 0.250 inch stainless steel line having an inside diameter of 0.190 inches. The line end was oriented at an angle of approximately 45.degree. and spaced a distance of 2 to 3 inches with respect to the shroud of a TF 30 turbine blade. The blade was preheated at 1,750.degree. to 1,825.degree.F by resistively heated filaments of tantalum alloy (Ta + 10W). When metallographic examinations were made, corresponding specimens coated with and without the admission of inert gas showed substantial increases of coating thicknesses on the shroud. The results of a number of tests are summarized in the following table.

TABLE I __________________________________________________________________________ RANDOMIZATION OF VAPOR CLOUD Chamber Pressure Coating Thickness (Inches) Coating Time Test Specimen Coating* (Torr) Airfoil Shroud (Min.) __________________________________________________________________________ 1 CoCrAlY 5.0 .times. 10.sup..sup.-5 .00360 0-.000125 16.0 2 CoCrAlY Ar, 7.2 .times. 10.sup..sup.-4 .00325 .00125 18.0 3 CoCrAlY He, 2.0 .times. 10.sup..sup.-4 .00338 .00063 15.0 4 CoCrAlY He, 6.5 .times. 10.sup..sup.-4 .00375 .00100 13.0 5 CoCrAlY He, 1.5 .times. 10.sup..sup.-3 .00288 .00113 15.0 6 CoCrAlY He, 4.5 .times. 10.sup..sup.-4 .00425 .00175 15.0 7 CoCrAlY He, 1.1 .times. 10.sup..sup.-3 .00450 .00225 15.0 __________________________________________________________________________ *Ingot Material: Air Melt CoCrAlY

It is to be noted in referring to Table I that tests 2 through 7 were run utilizing the inert gas leak of the present invention and thus should be compared with Test 1 which was run without vapor cloud randomization. It will also be noted that in Test 7 the outer shroud surface, which ordinarily receives no coating was coated to approximately 50 percent of the airfoil coating thickness.

The typical coating procedure has utilized a power setting of the electron beam gun at 21 kilowatts for the CoCrAlY material and at 15.5 kilowatts for the FeCrAlY coating.

What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in other ways than as specifically described.

Claims

What is claimed is:

1. A process of vacuum vapor depositing a protective metal coating on a metallic substrate comprising:

positioning a source of material to be vaporized within an evacuated enclosure;

positioning the substrate within the enclosure above said source in direct line-of-sight thereof such that vapor particles produced by heating said source material impinge on and coat line-of-sight portions of said substrate;

heating said source material to produce a vapor cloud of coating material moving generally line-of-sight from said source to said substrate and coating line-of-sight portions of the substrate;

introducing at low velocity an inert gas to increase the pressure in the enclosure to at least 2.0 .times. 10.sup.-.sup.4 mm Hg to decrease the mean free collision path of vapor cloud atoms to randomize their direction and cause coating of non-line-of-sight portions of said substrate, said pressure being sufficiently low to prevent sustaining a gas plasma discharge.

2. The method of claim 1 wherein the inert gas is helium.