Homogeneously Fine-grained Vapor-deposited Material In Bulk Form

Abstract

In one form of the invention a substrate in the condition and form of a hot titanium strip is advanced over a refractory mask in which are slots for admitting to a face of the strip the vapor of a crystalline material. Crucibles are arranged in a sequence beneath the mask to supply the vapor through the slots for deposition in very thin layers on the substrate. In another batch form of the invention the substrate comprises a fixed substrate sheet located above a slotted rotating shutter. Below the shutter is a crucible containing the material to be vaporized and deposited in the very thin layers upon the substrate. In both forms, the apparatus above described is located in a chamber which may be evacuated or contain a desired atmosphere. In each case the substrate is maintained at a temperature such that, as successive very thin layers of the vaporant are laid down, each condenses before the next is applied so that the latter will nucleate and condense without continuous columnar grain growth normal to the plane of the multiply material on the substrate. A number of layers are laid down until the material deposited on the substrate reaches a bulk thickness in the range of about 1 to 10 mils. The resulting homogeneously grained and layered composite is then stripped from the substrate.

Description

It is known that the mechanical properties of crystalline materials such as metals and ceramics depend on the homogeneity and fineness of their grain. For example, materials having fine-grained microstructure have better properties than those of coarse-grained samples of the same material. Thus such materials, due to their microstructures, have greater strength, shock resistance, corrosion resistance and other desirable properties such as behaving in a superplastic manner. The term metals as used herein includes alloys.

Ordinary vapor deposition produces fine-grained or microstructure in a deposit but if the deposition is continuously carried out to build up a desired thickness then columnar grain growth will occur normal to the substrate upon which the deposit is made. This has not been a problem heretofore because the deposits were generally so thin that columnar growth would not occur but when carried to any substantial thickness it would occur. However since the deposits were left on the substrate and used simply as coatings which were not required to have high strength, superplasticity or the like, no problem was encountered in the inclusion of the columnar grain growths.

The object of the invention is to obtain a laminated fine-grain vapor deposit material in bulk form, i.e., in the form of a ribbon, strip, sheet or the like of substantial thickness on the order of 1 to 10 mils wherein the microstructure in homogeneous throughout and free of any directional or columnar grain growth normal to the plane of the strip sheet or the like. This is accomplished by sequentially laying down very thin layers of the vapor, and allowing each layer to condense before application of the next. The thickness of each of the thin vapor deposited layers is in the range of one-quarter to 5 microns. The purpose of this is to interrupt columnar nucleation and bring about fine structure renucleation at each interface between layers. This, taken with the thinness of each layer, produces a homogeneous fine structure throughout the resulting multilayer composite on the substrate. Stated otherwise by interrupting deposition in regular intervals, there will be no columnar growth grain normal to the plane of deposition. After completion the multilayer deposit is stripped from the substrate and there results a laminated bulk mass of the material having the desired homogeneous structure. Other objects and features will be in part apparent and in part pointed out hereinafter.

FIG. 1 is a side elevation illustrating the invention carried according to a continuous process;

FIG. 2 is a plan view of FIG. 1;

FIG. 3 is a side elevation illustrating a form of the invention according to a batch process;

FIG. 4 is a cross section taken on line 4--4 of FIG. 3; and

FIG. 5 is a greatly enlarged section illustrating the product made according to the invention.

Corresponding characters indicate corresponding parts throughout the several views of the drawings which are illustrative and not to scale.

Referring now more particularly to FIGS. 1 and 2, there is shown at numeral 1 a strip of substrate material such as stainless steel, titanium or other suitable metal which will withstand the temperatures involved in condensing the vapor of a material selected for vapor deposition. The thickness of this substrate is not critical it being such that it may act as a moving support and preferably be coilable. The substrate is pulled from a suitable supply coil and into a windup coil. The coils are not shown. The direction of movement is indicated diagrammatically by the arrow 3 which also may be taken as a symbol for the drive means for the strip. The temperature of strip 1 is maintained at a desirable value by any appropriate resistance, induction or other heating means symbolized by the dart 5.

Below the moving substrate 1 is a mask 7 fixed in position and composed of a suitable material such as titanium, stainless steel or the like which will withstand the temperatures of evaporating materials to be mentioned below. In the mask 7 are ports 9.

Below the masks 7 is located a row of crucibles 11 containing the material to be heated and vaporized. Heating means such as focused electron beam heating apparatus may be used to bring about liquefaction, and evaporation is indicated by the darts 13. The rate of evaporation may be controlled by control of the heating means. This controls the deposition rate hereinafter referred to.

The entire arrangement described is located in a vacuum chamber indicated by dotted lines 15. This may contain a vacuum in range of 10.sup.-.sup.7 to 510.sup.-.sup.4 torr. In some circumstances it may be desirable to employ in the chamber 15 a nonreactive gas such as argon or helium or a reactive gas such as oxygen. The rate of deposition for titanium may be on the order of 10 microns thick per minute of titanium reaching the substrate 1 from each port 9. The temperature of the substrate should in this case be held at a temperature below 1,200.degree. F. In general a temperature is employed in the range of from 0.25 to 0.60 of the melting temperature of the depositing material in degrees Kelvin. Then in the case of titanium with a port width of one inch and speed of the substrate 1 of approximately 8 inches per minute, there will be successively deposited layers of titanium which will each be about one-quarter micron thick. An acceptable range is one-quarter micron to 5 microns. Each layer solidifies immediately before the next is applied. Layer after layer is applied until a bulk thickness is built up in the range of from one to ten mils. This is about 25 times as thick as is normally used in vapor-deposited film technology, which is on the order of if 1 micron or so thick, and in any event is not layered to bulk thickness for stripping. As each layer is deposited a renucleating process starts. It is by this means that there is prevented any columnar growth of grains normal to the plane of deposition. Finally the built-up bulk mass 17 is stripped from the substrate 1, as illustrated in FIG. 5. Ordinarily stripping is easy without further precautions but if needed to facilitate it, a parting compound may be used on the lower face of strip 1, as indicated at 21 on FIG. 5.

Referring again to FIG. 5, it diagrammatically illustrates the substrate 1 with the vapor deposited laminated strip of titanium 17 thereon. The dotted lines 19 indicate the direction in which the laminations occur, it being understood that there will be as many of these as are needed to bring the thickness of the layer 17 up to the bulk desired, such as in the range of from 1 to 10 mils. The six broken lines 19 do not indicate the complete number of layers obtained which may be many more than FIG. 5 indicates.

FIGS. 3 and 4 illustrate a batch process for carrying out the invention. In this case that is fixedly mounted a substrate sheet 23, which may be of rectangular form as shown in FIG. 4. Beneath it is mounted a crucible 11 employing heating means 13 such as above described. Between the substrate 23 and the crucible 11 is mounted a refractory rotary shutter 25 in which are variably open sectors 27. These periodically expose the underside of the substrate 23 to the evaporant from the crucible 11. At a deposition rate of ten microns per minute of evaporant reaching the substrate 23 a suitable speed for the shutter 25 is 1 revolution per minute in order to produce deposited layers each about 11/4 microns thick, assuming a 45.degree. angle for each of the open sectors 27. The process is continued until a bulk laminated layer is built up on the substrate sheet 23. Finally the bulk laminated layer is stripped as in the case illustrated in FIG. 5.

By procedures such as above illustrated, grain size within the planes of deposition can be controlled to be in the range of from 0.01 to 10 microns. The grains are homogeneous in size throughout the laminated bulk of the material. No columnar grains occur in the direction of any of the three dimensions of the sheet. As each thin laminate is laid down renucleation takes place at the interface between them so that there is on chance for columnar graining transverse to the plane of the sheet. In this manner structures can be developed with controlled microstructure yielding many desired properties of strength, superplasticity, shock resistance, corrosion resistance and like properties depending upon microstructure.

While each crucible in FIGS. 1 and 2 contains the same evaporative material as the others, it is within the purview of the invention that the materials in them may differ. The result will be a bulk, layered product in which the materials in various layers are different but characterized, as in the above descriptions by the absence of any columnar grains normal to the planes of the layers.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. The product comprising a bulk volume of laminated metallic material in the form of a sheet having a thickness in the range of from 1 to 10 mils and including a number of individual evaporated layers bonded to one another each of said layers having a thickness which is in the range of from one-quarter to 5 microns, the interfaces between adjacent bonded layers comprising planes of renucleation of grains of the material to define a fine-grain homogeneous structure throughout the bulk material characterized by the absence of columnar grains normal to the planes of the layers.

2. The method of forming a bulk mass in self-supporting sheet form of fine-grained metallic material having a thickness in the range of from one to ten mils comprising vaporizing the material, intermittently depositing successive layers of the vapor on a given area of a substrate and to one another in a timed sequence such that each layer is of thickness in the range of one-quarter to 5 microns, the time interval between successive depositions permitting condensation of each layer before the next one is applied thereto to interrupt columnar nucleation and to effect grain renucleation at the interface between adjacent layers.