The present invention relates generally to processes for coating articles by physical vapor deposition and, more particularly to processes utilizing electron bombardment energy.
Surface coatings have become particularly important in applications where the physical and chemical properties of materials must meet stringent requirements. One such application is the use of protective coatings to increase the corrosion and erosion resistance of interior parts of gas turbine engines. A number of special alloys have been developed for use as coating materials.
Surface coatings have been applied in a variety of ways. One method useful in coating with high melting temperature materials is physical vapor deposition in which material evaporates from a molten pool in vacuum and travels to the substrate where it deposits.
If the coating process conditions are such that vapor particles encounter few other atoms or molecules, then the vapor travels outward from the source along straight lines. The directional distribution of the vapor is described by a well-known Cosine Law which is applicable when the diameter d of the molten pool of the vapor source is small compared to the mean free path .lambda. in the vapor between the source and substrate. The result is that a coating of uniform thickness would be deposited on the interior surface of a sphere tangent to the pool and having a diameter large compared to d.
It is well known that deviations from the Cosine Law are observed at high evaporation rates in vapor sources which are heated by an electron beam in a high vacuum, such as 0.1 mT (millitorr) or less. In such cases, the relative deposition rate directly over the source is greater than that given by the Cosine Law. Departures from the Cosine Law have also been observed during high vacuum evaporation of material from resistance heated boats (see K. R. Carson et al, J. Vac. Sci. Tech., Vol. 7, No. 2, pp. 347-350, 1970). Similar departures from the Cosine Law have been observed in molecular beams flowing out of an orifice into high vacuum. Theories indicate that departure from the Cosine Law can be expected when the ratio .lambda./d becomes less than about one.
High rate electron beam evaporation has also been done in chambers where the gas pressures were in the range up to around 30 mT. (see U.S. Pat. No. 3,756,193 to D. C. Carmichael et al.) Collisions between the vapor and the gas result in deviations from the Cosine Law in that evaporated material is deposited on areas which are not in line-of-sight of the vapor source.
In many evaporation processes, the substrate subtends only a small solid angle at the vapor source and much of the evaporant is wasted because it is deposited on the walls of the chamber. It is desirable to increase the efficiency of such a coating process by collimating the vapor so that a higher fraction is incident on the substrate. Vapor collimation is particularly important when the evaporant material is scarce or expensive. Such collimation also allows higher coating rates or lower evaporation powers and thus reduces the consumption of energy in the coating process.
One vapor collimation method was described by Blecherman et al in U.S. Pat. No. 3,620,815. There the vapor was collimated by surrounding the vapor source with a gas manifold which directed a high velocity stream of inert gas toward the substrate.