Parts are sealed in an air tight chamber that is pumped down several atmospheres. A negative voltage applied to the parts attract the positive ion molecules bombarded from hard targets (. titanium, chromium etc.). This bombardment is a high-power electric arc that produces a highly ionized vapor. At the same time, a reactive gas (. nitrogen or a gas containing carbon) is introduced; it forms a compound with the metal vapor and is deposited on the parts as a thin, highly adherent coating. In order to obtain a uniform coating thickness, the parts are rotated at uniform speed about several axes.