A method for plasma immersion ion processing including providing a hollow substrate having an interior surface defining an interior and a gas feed tube extending through the interior, wherein the gas feed tube is hollow and includes a wall having a plurality of holes defined therein and applying tension to said gas feed tube by affixing a spring to one end of said gas feed tube and said vacuum chamber. The method may also include heating the gas feed tube to a temperature in the range of 50° C. to 650° C.; supplying a precursor gas to the interior of the hollow substrate through the plurality of holes in the gas feed tube and generating a plasma; and applying a negative bias to the hollow substrate relative to the gas feed tube to draw ions from the plasma to the interior surface to form a coating on the interior surface.