Abstract: A graded surface coating is reactively sputtered onto a tubular substrate by advancing the substrate in an axial direction through a cylindrical sputtering chamber in the presence of a sputter supporting gas. The sputtering chamber includes a cathode liner from which metal is sputtered onto the substrate. A reactive gas is directed into the sputtering chamber from a feed point outside of the chamber, whereby reactive sputtering occurs within the chamber. The reactive gas is induced to flow into the chamber in a direction counter to the direction of advancement of the substrate whereby, as the substrate is progressively advanced through the chamber and the applied coating gradually increases in thickness, the proportion of the reactive gas constituent in the coating increases relative to the metal constituent proportion with increasing thickness of the coating.

Abstract: A reactive sputtering process is described for producing a thin film of sputtered material on a tubular substrate which may, for example, be a solar collector tube. Uniformity of the composition of the sputtered layer at all points at the same depth in the film is achieved by preventing the movement of reactive gases through which the sputtering has already taken place, from one part of the zone of sputtering to another. In one way of achieving this result the reactive gas is swept from the zone of sputtering by entraining it in a gas stream flowing across the sputtering path. In an alternative way of achieving this result the rate of flow of reactive gas is finely controlled so that it is entirely consumed in one discrete area of sputtering so that there are no components of the reactive gas remaining which are free to move to other parts of the zone of sputtering and thereby change the atmospheric conditions in it.