Vapor Collimation In Vacuum Deposition Of Coatings

Abstract

In the processes for forming protective coatings on metals, particularly the nickel-base and cobalt-base superalloys, by deposition in vacuum, an inert gas cascade surrounding the melt is utilized to collimate the coating vapor cloud and reduce low angle deposition.

Description

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. 3,102,044, to Joseph, 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 loss 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 FeCrA1Y 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. Talboon, Jr., et al. entitled "Iron Base Coating for the Superalloys," Ser. No. 731,650 filed May 23, 1968, now U.S. Pat. No. 3,542,530. Another such composition is the CoCrA1Y 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. 2,746,420 to Steigerwald. 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 randomization or redirection of the vapor cloud. In one such method, a low-velocity, controlled inert gas leak, 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, has been utilized to randomize the direction of the metal vapor atoms thus permitting the coating of non-line-of-sight areas.

The present invention contemplates a vacuum deposition process which utilizes controlled inert gas impingement on the vapor cloud to collimate and densify the coating material vapor cloud. For this purpose, an inert gas is admitted at high velocity into the vacuum chamber through a gas cascade or multiorificed nozzle surrounding the pool of molten coating material, the gas being introduced at an angle to the vapor cloud to minimize low-angle vapor deposition and, in essence, to pump the vapors into a more concentrated vapor cone.

FIG. 1 illustrates, somewhat schematically, a vacuum deposition chamber with an inert gas cascade surrounding the molten pool and operating to reduce the angle of the vapor cone.

FIG. 2 is an expanded view of the molten pool area more clearly showing the relative position of the gas cascade.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown most clearly in FIG. 1, the articles to be coated 2 are mounted within a vacuum chamber 4. Inasmuch as the process is fundamentally line-of-sight, the parts are usually mounted to effect rotation about their individual axes, typically utilizing a pass-through (not shown) through the vacuum chamber to an external drive system. For minimum nonuniformity of coating between the individual parts, they are normally mounted in a plane of vapor isodensity 6, 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 said vertical. The parts are further positioned as close as possible to the surface of the molten pool for maximum coating efficiency but far enough removed therefrom to prevent coating contamination by splash from the pool. The pool 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 FeCrA1Y coating material, a mean height of about 10 inches has been found to be satisfactory.

There are a number of heating techniques which have been employed in the past in connection with vacuum deposition and heat treatment. A preferred heat source and that illustrated schematically in the drawing is an electron beam gun 8. The arrangement of the electron beam gun within the vacuum chamber is a function of design. In some installations, deflection magnetics have been utilized to direct the beam onto the surface 12 of the coating material ingot 10. A 30 kilowatt electron beam unit has provided satisfactory deposition rates with a 2 inch diameter ingot, the depth of the molten pool usually being 1/4-1/2 inch.

The ingot 10 is made movable with respect to the water cooled crucible 14 and is normally continuously fed into the crucible to maintain a constant pool height. This is important for two reasons. First, because the electron beam is focused on the pool surface, it is desirable to maintain the ingot level at the proper height. Secondly, because coating efficiency, composition and uniformity are very susceptible to pool height changes, a constant height relationship 16 between pool and parts to be coated is preferred.

The key to the present invention is the provision of a multiple orifice inert gas manifold 20 around the periphery of the molten pool, the orifices 21 being angled upwardly and inwardly toward the vapor cloud. A high-velocity inert gas stream, preferably helium, to reduce the incidence of a plasma gas discharge resulting in process instability, is utilized to narrow the cone configuration of the vapor cloud from its normal wide angle 22 to a more acute angle 24, with a resultant densification of the vapors. In essence, the inert gas stream reduces the scatter of the vapor cloud and thus reduces the material loss incident to low-angle vapor emission from the melt.

A number of tests were conducted with various coating materials and various substrate alloys. In one series of tests, helium gas was introduced at a line pressure of 17 p.s.i.a. through a 4 inch diameter ring manifold oriented concentric with the 2 inch diameter molten pool. Fifty-three evenly-spaced, 0.036 inch diameter holes 26 through the wall of a stainless steel tube, oriented at an angle of approximately 45.degree. with respect to the vertical, were provided for orificing purposes.

When weight measurements were made (corresponding specimens have been coated with and without the admission of inert gas) increases were noted in those tests utilizing inert gas admission. Additionally, spurious deposits of the coating material in low-angle locations on the vacuum chamber were significantly reduced.

The results of a number of these tests are summarized in the following table. ##SPC1##

Referring to table I, it will be noted that tests 2 and 3 were run utilizing the inert gas cascading of the present invention and thus should be compared with tests 1 and 4 which were run without vapor cloud collimation. In each case, three specimens were coated simultaneously, specimen 2 being the specimen located directly over the centerline of the 2 inch diameter pool, specimens 1 and 3 being located closer to the pool but offset from the vertical centerline thereof at an angle of 70.degree. with respect to the pool horizontal.

A vertical reference specimen located at an angle of 52.degree. with respect to the pool horizontal was utilized to ascertain the effect of vapor cloud collimation on the incidence of low-angle deposition.

It will be noted that in each case specimen weight gain was increased with inert gas admission through the cascade and low-angle deposition was decreased.

The typical coating procedure has utilized a power setting of the electron beam gun at 21 kilowatts for the CoCrA1Y material and at 15.5 kilowatts for the FeCrAlY coating. Control of coating thickness to .+-. 0.0005 inch at a designed thickness of 0.005 inch has been consistently achieved.

It has been mentioned previously that chamber pressures should be maintained at a low-enough value to prevent the incidence of a plasma generation in the process which results in a fundamental instability and a loss of control of the process. In the present process to reduce the possibility of plasma generation, helium rather than argon is preferred for vapor cloud collimation purposes.

The very important factor in the process, once satisfactory coating conditions are attained, is the close maintenance of these conditions as experience has demonstrated the relative criticality of the process to small variations in the process parameters.

While the invention has been described in connection with particular preferred embodiments and examples, these will be understood to be illustrative only. Numerous modifications to the constructional details, materials and process parameters will be evident to those skilled in the art within the true spirit of the invention as set forth in the appended claims.

Claims

1. In the process for forming protective coatings on metals by vacuum vapor deposition, the improvement which comprises: establishing a cloud of coating material in vapor form moving between a molten pool of coating material and the articles to be coated, said article to be coated being substantially within the line-of-sight of the molten pool; and at a plurality of locations peripherally of the vapor cloud, injecting a high-velocity stream of inert gas at an angle toward the vapor cloud to reduce the extent of low-angle vapor deposition and to redirect and

2. The method according to claim 1 wherein:

3. The method according to claim 2 wherein: the vacuum in the system is sufficiently low to prevent the generation of a

4. The method of improving the efficiency of deposition in vacuum coating processes wherein a high-melting point alloy is to be coated upon a metallic substrate which comprises: establishing a moving cloud of coating material vapor between the source of coating material and the substrate to be coated, said article to be coated being substantially within the line of sight of the molten pool; encircling the source of coating material with an inert gas cascade having a plurality of exit parts angularly directed inwardly toward and upwardly from the source; and admitting an inert gas at high-velocity through the exit ports to collimate the coating material vapor toward the vertical line-of-sight between the source and

5. The method according to claim 4 wherein:

6. The method according to claim 5 wherein: the pressure in the system is maintained at a value low enough to prevent the generation of a plasma upon the admission of helium to the system.