Method and system for coating internal surfaces using reverse-flow cycling

A method for coating the internal surfaces of a conductive workpiece having at least one opening comprising:

• connecting a biasing system such that said conductive workpiece functions as a hollow cathode;
• coupling a vacuum source to said at least one opening of said conductive workpiece to function as an exhaust;
• inserting a device into said at least one opening of said conductive workpiece, said device having a plurality of holes to enable gas flow to and from said device, said holes being arranged along an exterior of said device in a lengthwise direction of said conductive workpiece;
• flowing a gas through said conductive workpiece from said holes of said device to said at least one opening of said conductive workpiece to implement a first coating cycle, including generating a plasma within an interior of said conductive workpiece, said plasma having intensity that is adjustable by varying said biasing system;
• and
• with said conductive workpiece remaining in situ, reversing said flowing gas through said conductive workpiece from said at least one opening of said conductive workpiece to said holes of said device to implement a second coating cycle;
• and
• repeating said first and said second coating cycles to provide a uniform coating on said internal surfaces of said conductive workpiece, said repeating being adjustable for different said conductive workpieces of varying diameters and lengths.

A method of modifying an internal surface of a workpiece comprising:

• coupling at least one first opening of said workpiece to a gas source to function as an entrance and coupling at least one second opening of said workpiece to a vacuum source to function as an exit;
• connecting a biasing system so as to apply a negative pulsed DC voltage to said workpiece, including selecting a duty cycle such that:
• (a) when said voltage is in an “on” condition, a negative voltage is applied to said conductive workpiece such that positive source gas ions within said gas generated in an interior of said conductive workpiece are attracted to said internal surfaces and react chemically to modify said internal surfaces;
• and
• (b) when said voltage is in an “off” condition, said positive source gas ions are sufficiently replenished within said interior to provide uniformity in coating said internal surfaces;
• flowing gas having a surface modifying material through said workpiece from said at least one first opening to said at least one second opening to implement a first coating cycle;
• with said workpiece remaining in situ, reversing said flow of gas through said workpiece to implement a second coating cycle;
• repeating said first and said second coating cycles to provide a uniform coating on said internal surfaces of said conductive workpiece, said repeating being adjustable for different said conductive workpieces of varying diameters and lengths;
• wherein selecting said duty cycle for said “on” condition of said voltage and selecting a pressure drop across said interior of said workpiece include basing selections on an aspect ratio of interior length to interior diameter of said workpiece.