IL33785A - Apparatus for evaporation-coating with a metallic alloy - Google Patents

Description

Apparatus for evaporation-coating with a metallic alloy UKITED AIRCRAFT CORPORATION C: -32070 The coating of substrates by the evaporation ^&f coating alloys of two or more components (constituents) requires that an equilibrium be maintained between the solid ingot, the molten pool and the vapors which leave the pool. If this equilibrium is disturbed by any of a number of variables, i.e. ingc feed rate, changes in beam intensity and position of the pool within the crucible , the resulting coating chemistry fluc tuates so that the resultant coating chemistry cannot be reproduced reliably. Elimination of any of these variables will assist in producing coatings of constant and repiDducible chemistry and by a control of coating time, producing coatings of reproducible and uniform thickness.

One feature of the invention is the control of the level of the pool of metal or alloy in the crucible thereby assuring, when using a constant electron beam power, a more uniform rate of vaporization and a vapor of constant chemistry Another feature is the control of the pool height by a variable speed ingot feed which is responsive to a variable affected by the pool height. Because of the temperatures involved in the melting and evaporation of the alloy by an electron beam, it is necessary that the pool height sensing means not be affected by the beam. Accordingly, one feature of the invention is the sensing of pool height by a device insensitive to the beam and controlling the speed of the ingot feed in response to this device.

One particular feature is the maintenance of a constant evaporation rate and a constant vapor chemistry thereby assuring a uniform rate of vapor deposition and constant coating chemistry on the article being coated. This is assured by operating with a constant electron beam power and a constant pool height on which the beam impinges. In this way two of the variables that cause a nonuniform coating process are eliminated.

The copending applioation of Bleoherman et al, Serial ifigot feed, thereby controlling pool height.

Fig. 1 is a vertical sectional view, diagrammatic, through a crucible and associated mechanism incorporating the invention.

Pig. 2 is a fragmentary view similar to Fig. 1 showing a modification.

The arranganent is shown in a device for the coating of articles "by a metallic alloy in which the alloy is vaporized by use of an electron beam. In the drawing the vaporizing alloy is in the form of an ingot 10 extending upwardly into a water-cooled bottom-feed crucible 12. The ingot is fed upwardly as it is consumed by a variable speed motor 14 driving for example, through a reduction gear mechanism 16 to a pair of ingot feed rollers 18. The end of the ingot within the crucible is melted and vapori-ed by an electron beam 20 from a filament 22 forming part of an electron gun 24. A potential 25 is supplied from the filament to the accelerating electrode 26. The beam is focused directly on the end of the ingot and melts the surface of the ingot and then vaporizes it into the vacuum chamber 28 that surrounds the gun and the crucible. In the preferred operation the power to the gun, the electron beam energy is maintained constant so that the pool is exposed to a substantially constant heat input.

Above the crucible in a position for the deposition of vapor thereon is a workpiece or article 30, shown as a turbine vane, held in position on a support device 3 preferably equipped with a clamp 34- for releasable attachment of the workpiece.

The level of the pool of molten alloy may be sensed by a suitable sensing device which is, for example, a light sensitive cell 36 beamed toward the wall of the crucible at the pool level, the light radiation increasing as the pool level rises. The light intensity is measured and through a suitable computing mechanism 38 adjusts the speed of the in-got driving motor. Such a computing mechanism is conventional and does not require a detailed description. It will be understood that the motor speed is decreased as the light radiation increases to reduce the ingot speed and the speed is increased as the radiation decreases. IHao light measuring means is described and claimed in Bleohorman ot al,—Sorial No»— 8Q6.871 filod Moroh 1 j 1Q6Q.

The same type of device may be beamed directly into the vapors above the pool as these vapor atoms will be excited by the electron beam and will radiate light of an intensity proportional to the density of the vapor. If the beam energy is constant by reason of a constant power supply to the source of the beam, this density is directly related to the pool height and thus a satisfactory measure of pool height. Thro concept is also disclosed and claimed in tho abovo identified--application of Blgghegmam, et al, Sorial No.—806 ■ β?ί · With this arrangement the light sensitive device of Fig. 1 is beamed through the vapors above the pool and the operation is the same as above described.

Another dovioe for cont rolJLi-ag the s eed of th jjignti uses a radioactive material 40, for example, cesium in glass, in a shielding chamber 42, with the radiation directed by a tube 44 horizontally through the crucible and across the pool surface to a radiation sensing device 46 which may be an ionization chamber. This device measures the intensity of the radiation which increases as the pool level is lowered and the sensed variation in' radiation is fed into a computer 48 that determines the change in driving motor speed for restoring the pool level to the desired position.

This level sensing device may sense the liquid-solid interface level at the bottom of the molten pool which level is directly related to the top level of the pool so long as a beam of constant energy is directed onto the pool. Opposite to the device of Fig. 1, the speed of the ingot motor is reduced as the radiation is reduced as a reduced radiation represents a higher level of the pool within the crucible. Thus the motor speed is directly related to the intensity of the radiation sensed by the sensor.

Claims (8)

What we claim is :

1. A coating apparatus for applying a metallic alloy to a substrate characterized by: a chamber in which the substrate may be mounted, a crucible in said chamber, means for feeding a bar of alloy into said crucible, constant energy electron beam means for melting the alloy in the crucible and vaporizing it to provide a vapor to be deposited on the substrate, means for sensing the level of the pool of molten alloy in the crucible, and means responsive to said sensing means for adjusting said feeding means to maintain a constant pool level in the crucible.

2. Apparatus as in claim 1 characterized in that the sensing means is a device for measuring the light emitted by the vaporized material.

3. Apparatus as in claims 1 - 2, characterized in that the sensing means is a ray sensing means provided near a wall of the crucible substantially at the saLected pool level to be energized when exposed above the pool level.

4. Apparatus as in claims 1 - 3? characterized in that ray emitting means are also provided adjacent the crucible to provide the rays to which the ray sensing means are sensitive .

5. · Apparatus as in claims 1 - 4, characterized in that the chamber may be evacuated.

6. Apparatus as in claim 5 in which the crucible is a bottom-feed crucible.

7. · Apparatus as in claim 5» characterized in that the feeding means is a variable speed drive.

8. Apparatus as in claims 1 - 6, characterized in that radiation from the vaporized atoms adjacent the poo-i* and in that it includes a radioactive source, a radiation sensor, and means for directing the radiation through the ingot at the level of the bottom of the molten pool. 9· An apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings .